In a perfect world with a perfect coach yes. Unfortunately we have neither and top recruits demand getting their fav number to wear. I still crack up everytime I think of Batesy's story regarding Spurs and James's #44 and the recruitment of Daryl Bush.
It used to be only one player could have a specific number. I think when Meyer came, that changed due to the NCAA loosening the rules due to the amount of players. Regardless, the rules state that two players with the same number cannot be on the field at the same time. Most teams keep it simple, and have one player on defense and one player on offense with the same number since they would never be on a play together. Special teams is one of those times in a game where you could have players from offense and defense on the field at the same time, hence why guys throw over a different number over their chosen number. You could also see this with guys that play both ways if there is a conflict.
I'm pretty sure I saw someone in a game over the weekend, can't remember which one, that was shown taking off his jersey on the sideline and he had a jersey on underneath. The jersey he was taking off was obviously the one that was temporary for getting in on a play, likely ST. The temporary jersey he took off had his nameplate on the back. So, unless they have velcro nameplates, some players have a 2nd jersey already made up with their alternate number and name on it. I don't think I've ever noticed a jersey on top of a jersey before, or really thought about it.