I think it's a good idea to prompt for a birth place, but I do have some thoughts about it.
In the current revision round of my profiles, since May, I have actually entered a tentative birth place for all my profiles. But I wish that the interface would present a third option beside "Certain" and "Uncertain", maybe just called "Guess", indicating that there is no real evidence for it. It should also show up as such in the main profile page.
The way it comes across now, IMO, is that the contributor is pretty certain about the birth place, although without direct evidence. I've got some of those too, which rightly belong in the "Uncertain" group, but most are sheer conjecture and should be presented as such.