My gut reaction is that it looks a lot like centralization. And centralization almost always wins, and almost always for the worst reasons.

I can easily see peoples questions being buried/deleted/marked as "nofix" or other passive aggressive stuff like that.

I can easily see people who aren't on the project being less likely to have input than they would on SO.

I can also easily see a lot less discovery happening.

But all of these things that make a Q&A into yet another virtual fiefdom are exactly what will make it win out: it will be the developers own little fiefdom; they'll be there more.