For me, joining a forum is like joining a club: one per hobby. I might browse multiple forums in the way I might visit other clubs or read their newsletters, but when it comes to actively posting, I stick to a single locale.
It's just a question of time and effort. A good forum, like BoB, doesn't exclusively concern itself with a narrow treatment of the topic. There are, as everyone can see, sub-forums for clothing, food, sports, technology, and so on, all appropriately categorized as Related Pursuits; and of course all manner of sub-forums for pipe-related topics. If there was another equally good forum out there that I wanted to participate in, that would involve a lot of duplicate effort on my part, or else a bunch of unsatisfying copy-and-paste maneuvers.
Now if I had something to say, like maybe I carved my own pipes or blended my own tobacco, and I was really into it, I think I would probably participate on multiple forums within the scope of that passion. It's the difference between expert behavior and casual behavior. For this hobby, I'm definitely on the casual side.
I'm not sure about other pipe forums. I visit sites that aren't forums, but have forum-like setups: places like Tobacco Reviews or Pipedia. I get a lot of mileage out of them, but don't really feel compelled to post in them.
I've seen some forum posts concerning pipes at what I believe are general tobacco/smoking forums, but they haven't really grabbed me and I honestly don't remember the names. From my experience, I'd say, yes, BoB is the best pipe forum, which is why I chose to join this one.