It's mostly just to keep the same general 1 healer per 4 players. Having at least 2 rezzers is a good point, but you have the other 2 alliances that could raise your healer if they die. 1 healer per 4 characters is a decent idea, but it ignores the exponential strength increase of AE healing spells. That's ultimately the reason why it doesn't "feel right."
2 healers can heal EVERYONE twice as much as 1 healer. It's one thing if you have 2 healers single healing 2 different people, that fits in with the 1 healer per 4 players ratio. But given the fact that 2 healers can heal everyone at once for twice as much as 1 healer could, that's why it ends up being so easy in Alliance raids (Normal Raids definitely seem to be "more" balanced for that).
I honestly think that it'd be more interesting if healers had a debuff to AE heals in alliance raids to make up for that. I'm sure that'll never happen though.