Take all the specs you want. Especially if there are more tank specs. I would love more tanks to help with ques. Really though we only got a 4th spec because our Feral spec was too powerful allowing a tank to do almost as much DPS when not Primary Tank.
A few specs that could realistically have another spec - Shammy Tank, Lock healer maybe, Hunter Tank (though BMis pretty awsome). Monk caster etc etc etc.