I currently work with a hybrid build, currently more inclined to support than DPS.
Thing is, the border between them is very small, in that, when you take out a bunch of `situational` support skills to get higher levels on Ray and other DoTs, the damage difference is not that significant except on bosses. I have maxed Magnus and Oratio, a 4 or 5 (can't remember) Ray, and level 1 on the other DoTs, but those three I mentioned are all I use anyway if I go on a party as a DPS. I currently level exclusively in Osiris.
I do have Sanctuary and maxed Assumptio, which I barely ever use, unless I get into a party with a tank.
The reasons why no one goes for a Full Support build, I think, are:
1. If it's your main (like me), you screw yourself over trying to farm materials for your sets.
2. No one notices the situational skills, even basic Heal (which I have at level 1), and even if you say it's a lifesaver to raise Heal beyond 1 for me, the application is so limited (i.e. basic Heal has a cast time, ergo you'll rarely save a person with it, you'd probably go High Heal or Colo Heal instead.) that you're basically going to feel the wasted skill points.
But honestly, if I had a second priest or any alt for the matter (which I currently effectively don't, other than some storage alts and an iffy lvl 2x [not-ML] SM that I'm not levelling at the moment) I would definitely build my first one as a full-support. Because the thought of having all that utility is plainly amazing. However, as I don't, nor do I currently have the time and patience to do so, I would notice that the tradeoff for going hybrid instead is too little to let pass.