Most fights in Naxx at least require an offtank, to do it the "right" way. It's less for the MT to worry about, and helps the overall raid in case stuff goes wrong. We've had a few times where the MT dies, and the OT picked up, and turned a wipe into a kill. There's a few fights where one tank can do it all, so Dual spec, or toon swapping is what you'd want to do.