I would not call them a fail but there comes a point where you endanger the the completion of the run, for easy content like most dungeons that's all well and good but if you are not at least trying to push your characters to the limit for more difficult content and you wipe then you share in the blame.
As for healers not DPS'ing, this literally baffles me. The entire conjury story line teaches the merits of using all your abilities and not just healing. The Scholar quests have you healing and DPS as well, healing alone would never allow you to beat them. If the run is going just fine and you are sitting on nearly full MP and cannot be bothered to toss out at least a Bio/Aero here and there to speed things along a little bit then you are just being plain lazy. End of story.
As for new players wanting to choose to do certain mechanics, they need to check with the rest of the party to make sure everyone is on the same page. Because if one group is doing the fight one way and half the group is using a different strategy you're gonna have a bad time. People play differently and you should be open to new ideas, some of these "speed" mechanics actually make some fights considerably less stressful on new players as long as they can attempt to give it a try. I'm sure the devs never imagined a group would tank Leviathan Extreme with two Titan-Egis but I watched a group who cleared it and it looked like a blast but clearly did not follow the devs idea of how the fight would play out.
The important point is if you have a difference of playstyle with someone is to try and not act like a tool about the entire thing and work out some manner of compromise. I've seen people explode over helpful suggestions to make things easier for them and they scream. "DON'T TELL ME HOW TO PLAY!!!" The sooner everyone gets over themselves the better the community will be as a whole.