I cant be MT?

So I was told by my guild that I can't be a MT.

We had an awesome MT warrior but he xferred servers because a good guild accepted his app.
Now I was main OT. I figured I would be moved up to MT status and we would just recruit or gear up a new OT.

I was told that for most encounters having a warrior or pally MT is the best and that I couldnt MT these encounters. I think this is wrong. Any actual facts out there as to DKs taking a ton more damage or being harder to sustain than other tanking classes? Or anything that shows we are indeed acceptable MTs. I would love to have so hard evidence other than an opinion that we take too much damage.