Speaking from a 10M PoV: anybody who has the opportunity to run with monk and/or prot pala should do it, why wouldn't you want additional raidheal even if that's "only" 10k (or 50k, whatever)? It's the same question as if you can fill your last dps slot and already covered all revelant raid(de-)buffs, would you rather bring somebody with a strong offheal capability or just a plain dps (disregard their personality or possible loot dilemma).
If you can bring another melee or an enhancer, i'd go with the shaman any day. On Megaera heroic our enhancer heals for up to 300k hps during rampage with ancestral guidance popped and dropping a glyphed healing rain, how this can ever be balanced for 10M?
Similar problem with owl/shadow versus a pure ranged dps like a mage, tranq+symbiosis/massdispell+vampiric embrace will come handy more often (more or less on any fight) than a ring of frost. With the abolition of the "hybrid tax" a roster with lots of hybrids will regulary beat a normal roster. With the nature of most fights (burst damage instead of constant aoe) intelligent management of hybrids cooldown allow your team to bring another dps instead of a third healer, greatly decreasing the damage requirements per person.
Or turn it around, if you run with (fist-)mistweaver and disc priest you can easily add 100k raid-dps over a team of shaman/paladin/druid, how is this balanced? Yes, it adds the unique feeling to each class but to much diversity is just an uncontrollable element. Anybody who's willing to stack it will get great boni from this, especially during the progression phase. When you kill everything weeks (or months) behind world/server first, with +10 or more ilvl on your team you can run with any setup but if you still conquer for some more or less big records ... having a solid advantage won't hurt