As a tank, I love seeing my healer DPSing, it means I'm doing my job (mitigating damage). On that same token, if I'm taking pitiful damage over the course of numerous GCDs, and my healer isn't DPSing, it does tend to grate on me a bit. To put it another way, if a healer is going to play like that, it makes me want to not pop CDs, or put on more DPS gear. Why go out of your way as a tank to mitigate damage if a healer isn't going to capitalize on it?

That being said, I realize there is a skill cap to it. I main DRK, and the WHM in my static can actually spend the entirety of Neverreap's 3rd boss in cleric stance while I "solo-heal" myself by spamming Souleater.

Cleric Stance really is the biggest indication that the game WANTS you to DPS and it has an amazing ability (a lot of tanks would kill for something that swaps their VIT with STR and adds an additional buff to damage... pentamelds are expensive) enabling you to do just that.