If he's on a draw he can probably still call and expect you to give him your stack because it's obvious you have a monster, hence the minraise to keep him in the pot. If your opponent is observant and recognizes it for strength, then you just gave away your hand and let him make an easy fold. If he isnt on a draw well he probably would have called more than a 3bet anyway because he's playing top pair or whatever, so against you're outplaying yourself by not maximizing value.
If he's on a draw then you're being a sucker letting him get there cheaply. The problem with minraising is that if your opponent isnt observant, he'll call anyway because the bet isnt enough to scare him.
