Might be a factor sometimes but these days you can't keep a guy more than a year if he has playing asperations....so if you have a guy like Bo who is likely to be around a while, I'm thinking a good invited walk-on or maybe an "athlete" who can play QB is about the best you can get as back-up for at least another year.
If you sign 4 QBs, you are gonna lose at least 2 and probably 3 after a year or so. That might work if you have not identified your #1....but soon as that guy's name is announced, the #2 is probably gonna hit the trail....and these days, some of them don't even finish the season.....thinking about Joey and the guy at Clemson as recent examples..