Dual spec was a wonderful thing for me. I didn't realize how little damage I did as a holy priest until i doubled up with shadow. I don't generally fill the dps spot in raids (though I have a handful of times), and there are plenty of reasons for that.
One, I know the fights from a healing perspective. It's not that I'm not paying attention to the fight explanation, mind you. But the details of who to kill first, kill order, when to AoE and when to not do so... those are not details to which I've paid any attention. So even though, in theory, I know all the Naxx, VoA, EoE, and Sarth fights (haven't touched Ulduar yet), I am not the person who explains the fight to others. I know the details of what I should be doing as a healer. I know who is taking damage when, and I can assign healing duties. Don't ask me who to kill first.
Two, it is ingrained in me to heal. I've been doing it so long, I just can't stop. I'm not as bad on my shadow priest, because it requires me to pop out of shadow form. However, on my pally, even when I force myself to dismantle healbot, I can't resist healing at least myself, if not others. Especially since, as Ret, I get instant flash heals all the time. I don't really pay too much attention to healing or dps meters, but I know that my dps suffers considerably as Ret because... I keep trying to heal.
I'm awful at melee, I truly am. There's all this stuff going on on the screen and i'm just bashing my head against the keyboard hoping I hit something. Most of the time I'm out of melee range, and very confused. I can't figure out what's going on. See, health bars are relatively stationary. I keep my eyes glued to one spot. There's no dancing all over the place, having to worry about if I'm standing 1 inch one way or another. Maybe it's why I also suck at pvp.
And I have a deep, dark confession to make. I am a keyboard turner. ::weep:: I am doomed to failure at melee.
So if you should invite me along to a raid, and ask me to dps, just be prepared. If a healer dies, it doesn't matter how high my dps is (which, as Ret, is generally pretty high in a place like Naxx) - I'm going to start healing. I just can't help myself. I suppose from a RL mentality, this isn't necessarily a bad thing. I personally think it's good to have a few backup healers, people who can help out in a pinch. And in the same way that it's crucial you have an off-tank who is paying attention and can pick up adds without being directed to do so, I believe it's a good thing to have off-healers who can do the same.
Or maybe that's just me trying to justify my awful dps.