It is not possible to play without doing dps as healer? every day i find someone...

[Gullis]

While its probably not intentional, it can be kinda annoying when just want to do a quick roulette or dungeon run, and you see a player just stay there essentially afk, or spamming heals when everyone is at 90% + health or full, while you waste more time killing mobs way slower than you normaly would. Another negative side effect is that The longer the fights drag on, tank CDs will eventually run out, and the heavier you will need to heal, which can results in less tanking and shield/healing CDs for the next pull, futher slowing down the run.
Sure it won't make the dungeons go THAT much longer, but if you take every pull, and cut the normal kill time by 1/4 it does start to add up over the course of a full run.

Not that speed really matters that much, and most people won't expect a speedrun, but again, it can be annoying when you have to spend way longer clearing a dungeon, just because the healer refuses to dps for apparently no reasons other than, "i'm a healer, and therefore won't dps because reasons"
Obviously having a injury makes it a bit different, but then you should tell people that, instead of just saying "no i refuse to dps, because I don't want to"