It's so they don't have so much fatigue to rip through I believe. Communions above 4 Slaves really start adding up and since the fatigue is evenly spread you regenerate 20 fatigue a round (with 4) no matter what type of spell you cast. Plus remember that each slave gets the spell of their master. So if the Master casts Summon Earthpower, all the slaves under him have 4 Reinvigoration as well as the 5 normal fatigue healing from being over 100+ fatigue.