I don't know, it could still be useful in two scenarios. One, if you're casting a global that (for whatever reason) you don't expect to be dispelled (you're fighting someone with no astral mages). Secondly, if you're casting a global you expect to cast, have it dispelled, then cast it again...perhaps several times. For example, lets say I'm gonna cast Astral Corruption (that's the name for that blood global that hoses most other rituals, right?). Since that's gonna piss off every non-blood nation its gonna be a high target for dispelling, but fortunately I've got the blood magic site that reduces the cost by 60%. So, now when I cast it I can use the minimum number of blood slaves and get the 60% discount. When it's dispelled, my adversary has no idea how many blood slaves I put into it so he'll presumably pump a decent amount of extra pearls into the dispell, but guess what? The turn after its dispelled it's right back up (60% off!
). Repeat this a couple times, then put up a pumped up global enchantment at the point you've depleted your enemies pearls by leveraging your 60% discount. His next dispell will probably be a weak one, since you've been draining his gems faster than yours so even if he pumps up the dispell you should have the edge (unless he's just got a whole lot more gems than you do)