Most people who're interested in dual specs are going to be hybrids: Here's an example, a guy in my guild wanted to lvl a pally with the intention of healing at 80, the only way to lvl to 80 as holy is either to instance grind or get a lot of hard liquor maybe even some bleach. Most people will tell you to just go ahead and lvl to 80 as lolret then respec to holy and get to work.

Obviously you could make the excuse for respeccing holy for 5 man's along the way which may help a little bit but your spells will change greatly from say that one maradon run you did at 50 ish compared to the first time you step into heroic UK, also its arguable that questing beats out 5 mans anyway.

As far as the raid comment, I think thats a bit... elitist. I do know that my guild will be doing it for raids, like when you bring an extra tank for trash and him just sitting in the fight trying to dps as prot they're being told to respec to arms (I'm a rogue and I want the bleed increase :D) or for when we're a healer short we've had people respec to heal for us.

I imagine it could work the other way too, dks, wars, pallies etc who want to dps but would also like a shot at tanking can switch pretty quickly, I'm sure we've all sat there waiting for an hour for a tank.