Pld's have always had this problem in Front lines, because of the size of the match. Their usefulness really depends on the circumstances. I don't know how helpful this will be to the both of you, but here's a few things I've picked up on since the early Secure days:
- Crowd Control:
In Front lines, a Pld's CC does not have an immediately recognizable effect because of the sheer number of players on the map; so, only rotating stuns to reduce the enemy team's dps output isn't going to get you very far. In most cases, you have to look for specific targets who are worthy of interruption. Namely, 1) anyone who has a recognizable burst, 2) anyone harassing VIP members of your party (not only your healers but also your top dps), and, in the event your team is trying to murder someone, 3) enemy healers. You have to be picky about when and where your stuns will be useful. It's not a bad idea to occasionally take a pause and assess the situation. If you see a place where your stun might make a difference, go for it. Otherwise, you're better off running your burst on crystals and/or the enemy team.
- Party Defense:
Party defense is actually easier in Shatter than in previous modes, because there's less combat. In the old FL game modes, people were almost always trying to kill one another, so sometimes it was difficult to know when the best time for a Testudo or DV was. You could end up wasting it if the fighting ended early, or if you burned it too soon, etc. Most often you'd want to get as much utility as you could out of it by getting as many uses in one match as possible, but you also wanted it at the ready if you were under attack or were about to attack an enemy with large numbers. There was a trade off to each use.
In Shatter, this has changed a little. People spend most of their time avoiding fights, so it's totally fine to save your defensive CD's in most cases. In that regard, the trade off for burning a long CD like Testudo or DV is much less than what it once was. Depending on how aggressive your team or the enemy team is, you might not even need to use party defense CD's at all. If you want to be overly cautious about it, then you can alternate your DV and Testudo between shatter points that are being contested by an enemy. However, it is also completely fine to save it for moments in which you are undeniably under assault by an enemy team, providing a much higher level of defense for your team to stand and fight or make a hasty get away.
- Offense:
This is where things get tricky for Pld.
In past FL game modes, Pld's often acted as bait and snares. You'd dip into the enemy line until someone was dumb enough to attack you. Then you'd cautiously lead them back to the front (where your Dps were waiting), mark them as a priority target, work a Full Swing + GB DoT + CoS DoT, and then stun lock them while your dps melt their face. Most dps players are over-eager for the kill credit, so this wasn't hard to coordinate. It wasn't uncommon, however, for pre-made parties to have one or two Dps actually mark the Pld so they could keep an eye on them and jump whatever target they stunned. When Pld's weren't doing that, they were working on healer suppression, similar to what you probably know from Wolves Den or Feast.
In Shatter, your opportunities to do this are lower, because people don't attack one another. Even if the Pld snares someone, the dps will be too busy with crystals to help with the burn. That said, a lot of people underestimate a Pld's ability to "creep" on a target. If you don't know, creeping is what happens when people underestimate how much dps a Pld can output. It's a well known stigma that they're pathetically weak (because they are), but that also causes people to ignore them if they don't use their stuns. By the time a Pld uses their full rotation, it could be too late for the target to know what happened to them. It's tricky to do and fails often, but I've scored a lot of kills (even gaining battle highs and fevers) by slowly creeping on a target's hp without them realizing they were actually dying.
In shatter, the players who are least likely to realize what's happening to them are melee dps who are tunnel visioned on a crystal. I've literally stood shoulder to shoulder with mnks, drgs, and nins, and they've not realized that I was actually attacking them not the crystal. If they are isolated (either because their healer is elsewhere, or because line of sight is broken by the crystal), they will be forced to try and run. If they run (assuming you missed your stun), that's one less Dps on the crystal. If they don't, then they're going to die without immediate medical attention. Ranged dps are a little harder to sneak up on, because they tend to get jumpy as soon as they are within five feet of an enemy player. They can be worked on, however, especially if they're in a clutter of distractions. Given that they typically have lower Hp and defense, they're easier to kill. However, you do have to be quick on the stuns if they try to run, as they are more flexible in their movements than melee players. Enemy healers are typically the hardest to creep on, because they have a long standing history of being stun locked by Plds. Naturally, that means that they hate us more than anyone else, and they turn tail or execute a CC of some kind the second they see us coming for them. If you do manage to catch one, though, you can divert their attention from healing their team. This is only favourable if your team is on the offense, mind you, but it's something to consider.
- Leadership:
I almost forgot this one, because it applies to all Tanks, not just Plds. Typically, Tanks have WAY more time to judge a situation than other players. Their burst are less complicated. Their Hp/defense is higher, and their lack of presence is felt less than healers or dps. That means that, if you have the spine for it, you have more time to examine your map and take charge of the team by making accurate tactical assessments.
Personally, I'm not a big fan of doing this. To do it right means removing yourself from the battle and taking a lot of time to examine your map. You're basically sacrificing your personal play time for the sake of the team. That can be enjoyable in it's own way, of course. When you win with a team who is following your instructions it's very rewarding, but I prefer to be in the thick of things getting my hands dirty. You can't really do that if you want to lead, because you need to anticipate the enemy and call out when the best times to attack or retreat are. For as fast of a reaction as your team needs, you need to be faster to plan their movements; so, you often have to pull out of a fight early to check the map and make a good judgement.
There's also a lot of responsibility with leading, and any losses fall squarely on your shoulders (assuming your team is listening to you). That can be very degrading and demoralizing, especially if you get angry shouts in the chat box. That said, the impact you stand to have on the battle field is FAR greater than simply running headlong into combat. I can tell you right now that I am not the best leader. I make mistakes just like anyone; however, I can also say that the only reason I have my black ADS mount is because I got sick of consistently losing without leadership and decided to take up that mantel. I dropped it like a bad habit the second I got my mount, mind you, but I did get my mount. If someone else is leading, then by all means let them do it (the last thing you need is infighting, after all). However, if there is no leader and the team is losing, consider doing it yourself. Tanks are in the unique position to lead while having the least to lose on the team; so, its not a bad idea to take advantage of that, especially if your only role thus far as been to act as a sentry at your base.
Regardless, a Pld is only truly effective in PvP when they're doing all three of these things at once. You have to exercise good judgement on your CC, party defense, as well as offense to actually be a benefit to your team, and you only truly reach the top of your utility when your team (especially the dps) work with you and take advantage of your actions; so, active marking is often an asset. My best advice for you is to give it your absolute all. Pld's have lower dps than anyone else, so you have to work 3x as hard to make an impact on the battlefield. Whatever role you chose to be filling at any given moment, throw yourself at it like an angry rhino. Balls to the walls. That's the only way to roll as a Pld.
That said, there's only so much utility that you can bring to Front lines as Pld, and having more than one in the team is often not worth the trade off you take to dps or healing. If there is already a Pld in my party who is unwilling to switch, I typically will swap to drg or whm to cover for whatever deficiency is going to be felt the hardest.