I used to have that attitude. I just wanted to PvP when I hit lvl 70 and I was for the first week I hit it, but soon after I was getting asked to tank instances and I knew that my guild didn't really want a warrior in a 5 man that wasn't specced prot, or hybrid tank. Also, I knew that I could get more pug runs and essentially guild runs as a tank. So after some thinking I dumped some points into protection, got kicked from my guild for "wasting" my guild leader's time, joined a guild with my rl friends from work and now we have certain instances on farm status. BM is one of them. 3 BM's in less than 2hrs is pretty nice. Great rep!
Needless to say, I was one of those warriors who hated to tank, now I enjoy it more than I did. I found a way to have fun and be an asset to the rest of my friends and/or pugs that I group with. Most groups won't take me on a run unless I have some points into protection and so on. Now every time that I'm on, I get asked to tank, and I usually go if I'm not questing.
I guess its your ultimate decision on your spec, and if you're guild is ok with that, great. But you would be doing your guild a favor by speccing to be a tank. At least your healers would like it. I found out, by checking out warriors on my server, that there aren't too many tanks (that I could see) Since most of these instances drop tank and PvP Gear, I thought why not do my part until we all get geared, then I'll just go back to my PvP build and roll house. Not that I don't anymore, its just a little closer than I would like it to be, against other warriors at least.
Have fun with your build, but keep your fellow Alliance/Horde/Guild mates in mind too! Kara might be doable with out a tank, but it would go a lot smoother with one!!