I feel most issues are manufacturer specific rather than carrier specific; most, certainly not all. Having seperate forums for carriers would be an annoyance for me, as I'd feel the need to dig through all of them rather than just one. This is just my opinion.
Edit:
However, it might be productive for people to post their carrier along with whatever question, comment, or concern they are posting about.
Why do people want carrier specific forums? I'd like to know.