Is a tank all that I am?

I have been playing my DK quite a bit lately as I played one on the tournament realm and had a blast with him. He is specced Unholy dps and does a serviceable job in that role. I know from experience that any plate wearing class with the right gear can tank a regular dungeon at their level so I have been keeping tank gear just in case its needed, I like to help out. So I have been working him in Coldarra a little just to get the quests for the Nexus because they give out some pretty good rewards and I get asked if I am a tank. I say that I can tank and get invited to a group. Now I don't really mind but I already have a 80 pally tank who started as Ret and a 74 warrior tank who started out arms. I don't really want or need another tank, but both the dungeon runs I've done with my DK have been as a tank. I know at 80 it won't really be an issue because in Heroics, spec really starts to matter and I would have to totally change up my talent scheme if I was going to tank with him. While I will probably keep this option open as I like to tank and have been told by a few people that I am pretty good at it, sometimes you just want to melt faces you know what I mean?