Due to weaken soul, they conflict with each-other in a ways. It would help if they are assigned tanks for their shielding priority. As far as raid benefit, holy brings guardian spirit and stronger AOE healing, but that's pretty much it. As long as other classes are picking up the slack on the AOE, I would guess you are fine.
Shielding 25 people is great and all but impossible to impact unless you have a MTT, and Intervates. While PW:S is buffed, its not your only heal. Use Penance and Prayer of Healing to help with the absorb. Just shielding people left and right not only takes up time but a ton of your mana.