I'll agree with the requirement that dps should be careful about needing on tank gear, but because of inequities in queue times, I don't believe it should work the other way around:

If I'm tanking and I need that plate dps gear for my dps spec, you bet I'll be needing on it, without remorse or sympathy. Every plate-wearer with a dps spec can off-spec into a tank spec for an insta-queue and almost double his/her gear acquisition rate--and reduce queue times for everybody else by his/her personal increase to the tanking population. It's totally a win-win all around. And a rule that says that "tanks shouldn't roll on dps gear" tends to encourage dps to queue as just dps and not both tank and dps: exactly the opposite of the behavior we want to be encouraging.

I usually enjoy playing the dps spec more than the tank or healer spec. But if I queue as both tank[ or healer] and dps to reduce my (AND YOUR) queue time and the Dungeon Finder chooses me as tank[heals] instead of dps, you bet I'm rolling on that dps gear if my other, preferred spec needs it.