They keep their finger on the FD button in case of a wipe...
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Seriously though, it depends on the raid.
If they are the only tank or the highest level tank or the best gear tank then they tank.
In Hate for example they would offtank any adds that can't be mezzed (golems, drakes).
In Fear they are sometimes called upon to snare/fear kite mobs that dispell.
They are an additional means of FD pulling (well that maybe not so much after the recent changes).
And when you don't need any of that then basically they grab their highest damage non-procing weapon and pretend to be a 1-handed rogue.