Does this interviewing methodology mean that you only hire devs that don't currently have a gig or have vacation days to blow on the possibility of getting a job with you?
Wouldn't most people take a day off to interview somewhere? I know I wouldn't be comfortable ducking out for a few hours to interview — it would feel tense and uncomfortable.
Facebook, and I'm sure many other places, do full-day interviews. It's certainly not unusual.
Not always possible. One of the big annoyances when I was working for a company was that if I wanted time off it was two weeks notice. Doesn't work particularly well when the interview invitation tends to be more along the lines of, 'Hi, your interview date is next week.'