There's even a whm cutscene where a healer is refusing to DPS and it is causing problems and people are dying... healers ARE expected to throw out some damage too! If you are ONLY healing you are likely just standing around doing nothing a lot of the time OR you are overhealing needlessly. Of course it also depends on the quality of tank you get. If they aren't using defensive cooldowns at all, or are pulling ridiculous amounts of mobs in an untested roulette team your mileage may vary. Still, starting out a pull with SCH dots, AST dots, or regen and a bunch of holy spams is quite doable and helpful. It does take a while to get used to the speed necessary to pull it off, and keep people alive. Mostly just a lot of additional playtime.
That said, the SMN had no business demanding you play the way you wanted. Play the way your are comfortable, and try to adjust to what people expect as you can. Failing that, votekick, ignore, or leave. Usually I just ignore them completely like I don't hear them. Shame there isn't cross server blacklists.
