Seen from the Admin side, the problem is that members ever so often put stuff in the "wrong" forum. Not deliberately, rather because any splitting into groups and categories will be open for interpretation. The risk of misplacement actually increases if forums increase in numbers and are more narrowly defined.
From time to time, we move threads to a more appropriate place. My sentiment is that this over time is the better practice. We do want a 'lean' site without undue proliferation of options.