Mimic from Adamantoise had a world 5th clear with a NIN in the party.
A - Trick attack is powerful, if applied across averages, you get about a 1.66% increase to overall DPS. Not to shabby for a single skill that also has an extremely high potency attack attached to it. If applied strategically, I bet the real impact is somewhere north of a 2% effect on overall DPS.
B - NIN is very easy to play. It was not hard to master for anyone switching from MNK, and I would imagine easier for a DRG switching over as the buff/debuff management was largely similar. With TA being the only positional (for potency only), it is crazy good in those fights where the boss may be facing you at times, or there is a forced rear-only stack that flanking creates some risks (Shiva X, Titan X, T9 for a few examples).
C - NIN's dps is good, but I don't think that that is the problem. Nor do I think its the utility (I'm not of the opinion that DRG is fine as is, btw). This job becomes too powerful in its TP management. If they increase TP costs great players are going to stay great on NIN, good players with take a slight hit, and average and below average players will likely fall to pieces.
The combination of the fact that they can feed a MNK, and completely self-sustain with Invigorate for a VERY long time in a battle with their Mudra down-time, is really really good. That increases MNK dps, that increases BRD dps, that allows the NIN to worry about Ninjitsu order, Goading, silencing, stunning, AEing, Mantra-ing, CD management, and rotation (all things that MNKs have to do already) WITHOUT having the constant nag of TP starvation in long fights. And long fights are what we are all worried about here. If you make NIN a TP-intensive class that can be sustained about as long or slightly longer than a MNK, the class changes a little bit and the mental load becomes one in which great players will continue to push limits, and everyone else will either have to work really hard or lay off the accelerator to think about what they are doing.
Please SE - DO NOT nerf NIN DMG. They are a really fun and dynamic class to have in raids. I like playing it, but I think that my fate is to end up tanking. Even still - I'd really like to see NIN stay relevant for end game raids. I don't care if you buff DRG to MNK-level damage, just don't make NIN useless.