I wish the 120 prayer cape perk gave better benefits than just with training the skill. Like another poster stated, something that helps with prayer instead of the training itself. Perhaps, prayer regeneration (indefinite slowly over time), or 3 full prayer restorations per day, or slows prayer drain down by 75%... or something like that. Personally (just me) I think a prayer regeneration over time would be great. Let's say, 1/3 the speed of a prayer restore potions speed?
As some would think it's even pointless to go for 120 prayer when the 120 prayer cape offers the same effect as 99, what's the point of going beyond 99?
I feel (just my own opinion) that each and own of the 120 master capes should offer a different/better/unique perk than it's lower level version of 99. Make obtaining 120 more worthwhile.