I do it because I'm comfortable with the content. I know in dungeons not a ton of damage gets done unless a tank is making an effort to do large pulls. I know what will hit hard and what won't and there's no sense in keeping a tank at 90% hp when a mob will maaaybe take out 15% of a tank's hp before the mob dies. I start healing at 30% just to be safe but honestly with mob-by-mob pulls, my fairy alone can keep a tank around 75% hp while I just DPS.

It mostly depends on the tank's speed though. If you're pulling one mob at a time, I'm a DPS with a healer pet, but if you're doing large pulls, I'll keep you at 100%.