Good lord these topics are always a mine field. Having said that, this is probably the only game I know of where Tanks and Healers are expected to do more than what they are classed as. Not that its a bad thing as it just separates skill blah blah blah etc

It's honestly more of a game problem and a community thing. 2.0 wasn't too bad from what I remember and is how it still should be. It wasn't a requirement nor as expected as much but if you could pump out the DPS to help more then great!

So the first problem. We now have stupidly inflated health pools. Not a bad thing however the damage hasn't really increased that much (correct me if I'm wrong?). So essentially, we've gone up in health but the damage hasn't if that makes sense? (I'm aware our damage has gone up to a certain degree, but with the increased mob HP pool for enemies doubled compared to 2.0 too, it'll take us longer to kill things.) So healers can heal fine and then do nothing due to little damage going about and ONLY on target most of the time. Compared to say WoW, health is dropping down constantly on everyone in the group or the hits are that big on the tank, they have no choice but to heal requiring the healer to stay busy healing.

The other problem seems to have trickled down from raiding in savage. Due to the stupidly strict DPS requirements (due to poor QA testing I might add but dont get me started on that), Tanks and Healers had no choice but to DPS to help out. This as I said, has obviously trickled down into all content. It also doesn't help where content is mind numbingly boring at times in dungeons.

As we further progress into the expansion, then this should be easier to do due the inflated ilvls but sticking it right at the start is just stupid.

To sum it up, it's a design issue and the community have just responded to it to make things more efficient. Until it gets fixed, Tanks and Healers expecting to DPS as well as tank and heal is here to stay for the time being.