If it were a perfect world you're PF would always be on theirs, but basketball gets crazy, maybe it was a fast break, and your PG was the only one back in time to defend it, or maybe your PF got beat at the top of the key and the PG slid down from the wing to try and stop the drive but just fouled him instead. Change happens in the middle of the game and the players have to adjust as new situations arise, having your PG defend him is better then just letting him get to the rim unguarded