My whole time levelling I levelled as a Balance druid, and in my opinion, it's better for questing. When I reached Outland, however, I blazed through it in no time, and did very little questing. I was off-healing in instances virtually non-stop. I never made it past Zangamarsh.
So is a resto spec viable for levelling? At Level 72, very much so. In Azeroth, you can go weeks without ever running an instance, but once in Outland, they become pretty much /de riguer/, and running instances is certainly the fastest way to level. The same rule applies in Northrend to a fairly high degree, and once you hit the end-game, you'll be raiding. You've seen enough raids begging for healers to know what that implies.
Eventually, however, you'll want to save your pennies and dual-spec. In many classes, it's perfectly viable to get away with one spec. For some classes, though, like Druids, Paladins, Shamans, and Priests, especially, dual-spec is pretty much a necessity. If you have the option to heal, you should keep it on hand. Somebody mentioned the new dungeon grouping tool. A healer seldom ever has to wait in that queue more than a minute or two. At the same time, you'll eventually wind up doing dialies for reputation (and cash), so being a boomkin when you want to be is very handy, too.
The question you should be asking yourself, I suppose, is this: "Should I dual-spec now or later?"
