As long as I've been playing WoW (3 years now) the loot rolls have always been need on your main spec (the one you're playing at the time you're in the instance), only need on off spec if no one else needs. I've noticed that since Cata came out, there are many people who need on the spec they aren't playing at the time; needing against another person who is playing that spec - for example, a dps pally needing on a tank item and the tank atm needs it too. Why should a tank have to keep going thru the same instance over & over til they get that item? If a dps wants a tank item, then queue as a tank. The same goes if a dps needs on a healer item or a tank needs on healer/dps item. Whatever spec you are playing at the time should be the gear that you need on.
I think Blizz should fix this and take away need for an item when you're not in that spec. Play the spec you want to need gear in. It's only fair to everyone else who goes thru an instance multiple times to get that piece of gear then has someone else ninja it from them. Just because there is a need button doesn't mean you should take that item from someone else in the right spec.
