While this certainly makes sense, I've never read about programs doing this. On the other hand, many programs are explicit on their sites about accepting more than they have room for. Do you have any sources?
From what I have read, if a program wants a class of 4 or 5, then they will send out 10-12 (sometimes up to 15) acceptances because, as someone else has already stated, they will be competing with other programs for these applicants.
Interviews can be thorny for ad comms because of implicit biases with respect to ethnicity, gender, and socio-economic class, among other things. Speaking generally, interviewers tend to go with people more like themselves.
I would imagine it depends on your area of interest. Given that you're doing formal epistemology and logic, it might be a plus to have a strong background in math. Of course, this isn't going to necessarily compensate for other weak portions of your application (if you have them).
I noticed on U Chicago's website, where they discuss the admissions process in detail, that they expect more out of the writing samples from applicants with M.A.'s than they do those applying straight from undergraduate. I wonder how common this practice is.