Exp loss/deleveling doesn't foster a particularly helpful environment. I had a terrible time getting help for things in FFXI if there was a chance that the people who didn't need the mission/fight might die during it (until SE removed the death penalty for mission BCNMs, anyway).

I was really happy when FFXIV came out and had durability loss instead of exp loss. I can always pay people for the repair costs they might incur from helping me, but I can't pay them back lost exp.