Immerseus is one of the only fights that I think becomes less of a chore (I wouldn't call it "hard" over a "chore") since more healers actually allows more area coverage for puddles later on = faster killing. It's one I'd almost unequivocally recommend 3 healing even if stupidly overgeared.
I think sometimes the decision simply comes down to: 1) Are our healers comfortable under-healing? (If your healers are bad, obviously you want to 3 heal as much as possible), and things like 2) Is our healer comp good for this fight? (Disc Druid, for that matter, is strong), and even might just go to 3) Do we have mainspec healers we want to bring in for loot on an over-geared farm boss? Basically in many cases, it's really a raid leader's call for the "possible 2, possible 3" ones.
(*) Yes, I understand things die quick if you "massively overgear" a fight. But also 2 healing is not very difficult on most fights if you "massively overgear" it either.