There are so many ID request postings that, I feel, subforums would only mean that instead of one main ID forum with many pages there would be several ID forums, with many pages.
I think that the logic behind the 'sticky' ie area and location in headings, if, when used correctly, is fine for getting ID help. People with enough knowledge of a geographical location and its birds, and who are willing to help others, only have to flick through the postings and respond to ones suitable to their knowledge.
My opinion is that if it's to much work for people to move from 'page 1' to 'page 2' then it will be even more work to open sub forums and subsequent pages.
We musn't forget either, that birds are not bound by locations and geographical sub forums could hinder, rather than help, with the ID of unknown migrant species.
If I was to post an ID request for a North American wader, for example, I would get loads of help from people outside of the US, some of whom just quoting stuff from their 'shorebirds' croom helm's. The best help, for me, in that situation would be advice and assistance from someone who sees the bird regularly, ie a USA person.
Geographical sub-forums could hinder that kind of 'first hand' help.