@Mike
I find it more challenging, but still doable, but also see no reason not to make it simpler and easier. I healed without one through around Mists, at which point someone in my guild/raid was talking about it and recommended it.
The major advantage to me is that I can keep all of my offensive skills on my main hotkeys, instead of having to use my secondary keys for either that or healing (I prefer using my secondary keyset for profession skills, eg, fishing, surveying, mounting, prospecting, milling, disenchanting…) and I never have to unfocus the boss, which is particularly helpful when you’re playing something like a Disc priest and are trying to do both at the same time.