One of my current projects is to try tanking on different classes in WoW, just to get a feel for how the mechanics work. I have tanked a few instances on my Death Knight so far, and since I have a perfectly good level 80 resto Druid alt, she’s next on the pimping block.
Now the challenge of gearing up an existing alt for a new role is how you prefer to collect your loot. Do you queue up as your current overpowered self, to make easier dungeon runs both for you and your whole group? Or do you queue as your severely undergeared and inexperienced new role instead? If you pick the former, then you have to also decide, will you roll need for your offspec gear? If you pick the latter, how do you feel about the possibility of being bitched out by the rest of your group for both poor gear AND inexperience?
I had an amusing experience when running normal trial of the champion (to pick up a tanking trinket for her). I was queued as a healer. The trinket drops. I hit need, because that’s the only reason I am there. It is won by a Death Knight, who spends the next minute haranguing me as a ninja.
The punchline: He wasn’t tanking either. He was queued as dps.
There is no off-spec, there is only dual-spec.