I've played around as a DPS, healer and tank at level 50 and highly recommend everyone do the same before settling in on a favorite class. I'm definitely a much better healer and tank for having DPS'd and vice-versa etc.
However two common ideas are always brought up on the forums and it bothers me because they're just blatantly untrue.
1) Tanks (or healers) know the fight best.
I take umbrage with this as I find myself far more aware of fight mechanics as a DPS than either a tank or a healer. I suspect this is because healing and tanking inspire a kind of tunnel vision, since a large amount of your attention is devoted to looking at the party/enmity list and not the actual battlefield, and also because reaction times need to be faster on average.
Now why do I say this? Tanks are supposed to take damage and healers can prioritize their health over anyone else in the party (minus the other healer). A DPS has no fallback; learning boss patterns, when and where adds spawn, fight mechanics and phase changes is critical to avoid taking damage and risking a KO because the healer is too busy elsewhere. Healers can also take up static positions in many fights, whereas DPS are usually constantly on the move, chasing adds etc. and going in and out of "safe" zones, a hazard that healers don't have to deal with. Why do DPS have to experience these hazards? Because we are also usually tasked with things like feeding snakes (T1), feeding worm (Copperbell HM), killing eggs (Pharos) etc. which make us aware of boss stacks, hidden timers, etc. Additionally, I frequently find myself running all over the place chasing adds, especially in CT, and twice as much if the other DPS isn't doing their job and I have to run halfway across the map BECAUSE DAMMIT KILL THAT NAPALM @$#$@! /rant.
Anyway back on topic, I don't mean to say that healers are lazy and stand in one place spamming Physick/Cure. But I find my movement is much, much lower as a healer than a DPS and I can often find a "safe" spot to remain for the majority of the fight. This is almost completely irrelevant for a tank, obviously.
2) DPS derps/bad DPS aren't noticeable.
Now there are many DPS checks in the game where if the DPS is too low you wipe, but that's not really what I'm talking about. I'm talking about DPS who don't follow marked targets and pull aggro. Or DPS that's too low on trash mobs and require me to blow CDs to survive what should be a fairly routine tank and spank. Reading the forums and being subjected to the whole "ehh DPS whatever" attitude I was completely unprepared when I started tanking and realize what a pain in the a** a bad DPS could be. After running a dungeon a few times, you definitely know when you have good dps and bad dps. I concede that their influence is less noticeable, but it isn't invisible. A tank or healer screw up is like a heart attack or stroke, instant and obvious. Bad DPS is like a cancer, maybe it takes a while to notice but it will definitely kill you.
Anyway, please stop spreading these misconceptions. It's probably the reason I frequently run into SMNs who don't use dots, BLM who think they can get away with spamming Blizzard, and DPS of all classes who blatantly ignore fight mechanics because they think they are somehow absolved of all responsibility to do more than just button mash.
P.S. Everyone should really try playing every class at lvl 50. I find myself enjoying healing a lot more than DPS which surprised me. I'm sure there are a lot of people who would actually find other roles more fun than the one they originally chose. Also it really does round out your knowledge about the fight, thus making you a much better player. Also then you can think to yourself, "wow, I didn't realize how much easier/harder it is on the other side" (I'm looking at you Titan).