I was wondering the same when comparing my self to the other 2 top resto druids in my realm. I tried my self going for a high spirit build for some fights and I really liked it. I could toss a lot more rejuv + WG on heavy aoe fights like elite protectors, tsulong (night) and garalon. I'm assuming the heroic versions of these fights demand even more mana, I wouldn't want to do them with anything less than 10k spir.