I don't see Holy Spirit being used often in MT'ing, as you are seemingly not able to block, parry and/or dodge attacks during the cast.
Sure, 1.5 cast time seems to be rather quick, but burning the whole mana bar down in one go leaves you with 7.5 of the 12.5 seconds on reduced defense.

But being a OT with a WAR seems to be his ideal place - Burning Req+HS and applying Interventions to the WAR.