The problem is that most people want to see something besides DPS from a DPS class, especially since in most outland instances you want at least 2 crowd control (and on heroics an off-tank helps at times). While druids DO have hibernate, most people dont realize it - and honestly not too many instances require it. Thus, each of the other specs brings "more" to a group than a moonkin:
A resto spec brings healers, which are almost always in short supply. Sure you can't DPS and you may take forever to solo, but you'll find groups faster than Gilligan will find a way to ***** something up and you bring the HoTs which no other class can come close to rivalling.
A feral spec brings DPS, but also provides an off-tank if the gear level is fitting. Tanks are in as short supply as healers (sometimes more, sometimes less, but overall about the same) and a feral druid is considered by a large portion of the players to be the best off-tank in the game, while still providing comparable DPS if the tank isn't needed.
Moonkin can provide a lot of things that feral and resto don't, but because those specs fill all the important roles (healing, tanking, and DPS is covered by those two trees) moonkins sort of sit back. There have been instances I've done where I've said "no" to a moonkin for no other reason than we simply needed someone with CC, not just another basic DPSer.
However, despite the stigma, I'm respeccing my druid to moonkin as soon as I can get a set of balance gear. Mainly because my guild has 3 other feral druids, and we'll have 2 resto druids soon, so I might as well go the spec nobody else is. I'm even taking a hit in gear level (was in high blues and about 6 epics in my feral DPS set, dropping to mostly blues with a few greens and purples) to do this, because I think overall having a large variety of specs will benefit the guild.