Because 3-2 is a zone, it's impossible to put your 'best' defender on him. You do get a bonus to OD, definitely. But you may end up being unlucky with your worst defender in your 3-2 zone defending him on a 3 point shot for example. If you want him marked out of the game at all times, you'll need to play man to man.
The main advantage of 3-2 against an outside offence is when a PF steps back to take an outside jumper, a guard/SF with high OD will rotate to defend him, as opposed to M2M defence where your PF/matchup will guard him.