Eh, "MT" has grown to be in some ways a pretty much ceremonial position but its not as useless a term as I think some people make it out to be. It is true, however, that having a well geared stable of tanks is better than having one main and a bunch of scrubs. If a tree falls on your MTs house, you should still be able to run the raid. There are and will be fights in the future that will require multiple tanks to be well geared and well played.
But in the context of the current raid content, your MT is the one who will kite Anub, tank Faerlina, tank Maexxena, soak HS on Patches, etc, etc.
I hope I am coming at this in a non biased fashion as I am set to be the "2nd tank" for our 25 man Naxx starting Friday night. I'm fine with that because the nominal MT is a good friend and we compete/push one another in a productive manner.
your hat may be nice, but I have the little white tank top that says Legendary right across my boobs. I win. (or more correctly, H wins)