[Baalz]

Yep, obviously this is a great oversimplification, but if you do accept my previous suggestion that 10 gems = 2 extra combat casters (which makes sense if your mages are constrained by your number of castles and you have a research target) then an outfitted banelord is more like 6 extra casters. Obviously everything has its use, but 6 banefire/enslave mind/gift of heaven/thunderstrike spamming mages can most certainly trump a decked out banelord in lots of situations.