First and foremost, imo, fragmentation: if you have a forum which is only frequented by a handful of persons, the answers which you will get in that forum will be from that handful of persons. A lot of knowledge that is there in FT will not benefit the poster, because many knowledgeable persons will not frequent that forum. And the answers which ARE posted will equally only benefit a handful of persons even though they may be useful to the wider FT community.

The problem is compounded by the potential snowballing effect, as A will argue: well, look there is already a forum on X. So why not a forum on Y? sounds familiar? It should: there are several posts of that kind in every thread that proposes a new forum on something marginal to FT. You could end up with a very large number of tiny fora with a minuscule audience to the point where it is completely unmanageable from a user point of view, as the visiblity of a particular forum diminishes as the number of fora grows.