As a WHM, I can confirm DPS is usually required and not impossible for certain phases. (Patch 2.1!)

Phase 1: Holy x 5
- SCH solo heals
Phase 2: No dps
Phase 3: Mana recovery & Stoneskining!
Phase 4: Holy x 3
Phase 5: SCH can apply DoT's
- WHM usually recovers MP
Phase 6: HEAL MODE! (We have 2 Dreads and WHM heals MT)

If they kicked you for not DPS'n though... Poor damage dealers are poor.