I'm running bear + 3 moonkins + tree. I've tried it both ways and while you can indeed do 4 casters throwing out the oddball heals every now and then, it's much less efficient than just having a tree (by efficient I mean being able to maintain both heals and DPS over a really long fight). Plus you don't get a spammable AoE heal unless you get wild growth, so you're relying on a 10-minute CD for each aoe heal. Not a great option, especially if it's an extended fight that needs a lot of minor aoe healing throughout the fight. Honestly, though, that's mostly theorycraft as I didn't really try the 4x DPS + ghetto heals for long, and I've only done heroics and L80 instances with bear/tree setup.

Rejuvenation (identical gear; ~1750sp): moonkin = 1050ish/tick; resto(in caster form) = 1338/tick; resto in tree form = 1543/tick
[edit] Same druid, same gear. With moonkin/imp. moonkin aura = 1800sp; w/o moonkin = 1500sp. That's significant.

Another point to consider. Caster = 3237af (-17.5% dmg taken); Moonkin = 14637af (-49% dmg taken). Granted it's physical mitigation only, but think of it like having a mage with no AF buff, no ice block, no frost barrier and no mana shield. VERY squishy indeed. Example: in heroic Gundrak, when the rhino gores one of the moonkins, he'll take 30% dmg, which I tend to largely ignore. If the tree gets gored, it's like 65% of his health BAM. Huge difference when you're trying to gear up in terms of being more forgiving on mistakes.

A final point: glyphs. Can glyph both heals and DPS for the most part. You could but you're missing a sizable bonus either way there.

Ya know, I might just have to go respec my guys to some hybrid balance/resto spec minus moonkin and try it just to see what happens. My curiosity is piqued now.