Sorry to be late on this, but...

Echoing others, nsx hit the nail on the head. If I had to answer the question without the benefit of nsx's post, I'd say:

I'm generally a fan of creating new forums. The reason is that there's rarely a request for a forum with no need whatsoever. I don't see too many requests for a forum for an airline no one flies or a hotel no one stays at. Once the request is in, if it's an airline (or other program) that generally is well-known, I believe that having its own place helps many people, but especially newbies. Everyone on FT was once a newbie, myself included, and making it easy to find where to post (and not get your post moved or get flamed!) encourages them to stick around!

A perfect example, IMO, would be the somewhat recently created Iberia Plus forum.