About 40% of my passengers want to sit in the front. I have no problem with it although I have a Corolla so it can be a little compact. If they are bigger then it means sometimes when I try to change the AC settings I might hit against their leg or something. That can be very awkward so I try to avoid it! lol
A trick I saw here:
Move the seat all the way forward and put some highly visible things in the front seat in order to discourage people from sitting there. Most people will just sit in the back.