I agree with this completely.
The whole "I shouldn't need to DPS because the content is easy" argument makes me chuckle quite a bit..
If it's so easy then you also shouldn't need to heal much. So you might as well DPS isntead of standing around, in order to speed things up and help yourself and other people do another run or get on with their other in-game needs sooner.
I agree with the OP, you do you, and I won't rage at you for not casting offensive spells as a healer.. However I also won't commend you if you don't put in that extra effort where you can.