@Clevins:

I actually think there is a LOT of merit to the notion that pure DPS classes will find a flow-on benefit from dual spec in the form of more healers and tanks available.

My main is a pally tank. I tank normal and heroic raids and have very nice gear. I also maintain (by which I mean fully maintain - gems, enchants, everything as good as it can be) a healing set.

In the wee hours of the morning, someone is always looking for a tank or a healer. it just happens to be that when I am specced tank, everyone wants healers, and when I am specced healer, everyone wants tanks.

It's no small thing to respec. It's annoying to have to burn my hearth to get back from respeccing, it's annoying to re-glyph, and it's annoying to re-set action bars.

If I could have two specs to swap in and out of, you would see me healing when healers are needed, and tanking when tanks are needed.

I am off server time, and I gave up playing my DPS toon as my main because of the difficulty getting instances in my main play time. As a tank I can usually find work.

And finally, there is the simple numbers game. Let's say a guild has 2 teams for Naxx 10. They want to move to Naxx25. Each of their Naxx10 groups has 2 tanks, 2-3 healers, and 5-6 DPS.

If possible, you don't want to PuG tanks and healers - you'd rather stick with your known-good players. DPS is not as critical, and you can carry more of them with good tanks and heals. And, DPS is a heck of a lot easier to pug than heals.

Wouldn't it be great to turn one of those extra tanks into a reliable healer for your 25 team? Awesome for the raid, kinda sucky for the tank/healer who has to respec all the time.

With dual spec you won't be tempted to run with 4 tanks to keep them all happy. One of them will happily swap because it's not so much a pain in the butt and bank account. So there's one more DPS spot opened up in the raid.

Maybe you're the go-to DPS for your guild. You're in high demand so you never have to pug anything and there is always a raid spot saved for you. Then you get nothing at all from dual-spec except for an easier PvE/PvP swap. But if you do pug, or are on the fringes of your raid group, dual spec will benefit you.