Well, if you do the math, its not hard to see generally what shaman's are "prefered" to do in endgame play.
9 classes X 3 specs per class = 27 different setups.
Of the 27, Only 3 are tank focused specs, 19, are dps focused and 5 are healer focused specs. Dps is a dime a dozen, and for the most part either of the 19 dps specs are inter-changable between themselves. But, trying to find a healer, or should I say someone that CAN heal...is a much rarer breed. So, due to this fact shaman's get shoe horned into healing, due to the NEED.
Typically, if there's 2 slot left in a party/raid, and there's you (a shaman) and a rogue/lock/(fill in the blank dps spec) to choose from. Guess what the odds are that the party/raid leader is going to pick you as a DPS, and "wait" for a "healer" to show up? :p
You guessed it, they're going to pick you for healing, and grab the dps class (there's ALWAYS dps LFG). Hell, you could have been even a "A" class, top notch DPS, even better than Mr(s). "X" dps class. But DPS person "X" bandages will NEVER come close to the amount of healing that you can do, even when your not "speced" for it as a shaman.
Unlike Druids, we're only a 2 dimentional "hybrid" spec....Healer OR DPS. Druids are a full 3D hybrid...Healer/DPS/Tank. By the #'s, it would be to no surprise that Druids "tend" to tank more simply due to the fact that there's less spec's based on tanking, then even compared to healers, which by default...over shadows the possiblities of their DPS roles.
Shaman's on the other hand, since we can only fill 2 roles in a party/raid...and one of thoughs roles can be easily filled with A LOT of other options...guess what happens :P
Now, this being said...I am a 70 Enh shaman, and have been one for a while now (granted I have played all specs at one time or another for about a year each). But, my choice on Enh is a intentional one, due to the fact that since I play in a "odd" time zone, and difficult time schedual, and of late raiding/partys are nearly impossible. So even if I spec'd resto, it wouldn't be doing me or anyone else any good :P I spend most of my game time farming, and resto SUCKS for that roll. Thus, Enh was a obvious choice for me. Granted I used to raid TONS as a Enh, but I had a more unique situation in my guild that allowed me to do so. Few of the factors was I was formerly a Resto shaman, so I have a nearly complete Epic set of healing gear, and the veteran experiance of a healer role...thus I'm more than compitent to fill that role if/when needed, and I was called upon frequently to do so also.
Secondly, if/when I was able to dps, I tended to be #1-3 on the DPS meters, AS WELL as Top 3 on the healing too. So I proved my worth as being a truly active hybird, taking care of all bases. Not to mention I'm a "nice guy" on TS, and I can shmooze myself into anything with my charm -_0 But thats just me, and my situation...can't speak on behalf of the rest of the shaman lot.
But, generally speaking and looking at the math from what I mentioned above, be ready to heal, and you might have already noticed from playing currently. That situation isn't going to change ever, unless other classes are given more healing roles :p And I don't see that happening anytime soon :p