All three types of healers have there place. On raids, almost all main tank group use Templar, Warden and Defiler/Mystic. We personally use Templar(myself), warden and Defiler.
Group wise, I think any class of healer will work if the healer knows what they are doing. Templars buffs give the most HPS. But healing wise, like I said, if played right in the grouping environment, any of the classes can get the job done.
Ralalron - 77 Templar
Dendrum - 77 Berserker
Thonatai - Nektulos