The key to being a good defender is to give enemies a basic choice: waste attacks against me, or try to attack my buddies and be punished horribly for it. That way, no matter what the DM does, you win.
So, you've now put the enemy in the position where it's bad to attack the defender and bad to attack someone other than the defender. The good option is to not attack, but to run. Yet, the monster doesn't run. Why? Because the DM is playing them like suicidal lemmings and serving them to you on a silver platter in fights that the players are very likey to win, the monsters very likely to lose, but they fight anyway.
Hence being a defender isn't about making the choice of attacking another party member less appealing to enemies because it's already completely unappealing since fighting at all is a guaranteed death warrant. The role of defender, if there is any quality that could really be said about it, is probably damage mitigation.
No sense relying on the DM doing something dumb when you have the tools required to win even if the DM plays smart.
The DM playing smart means having the monster run and get enough allies that it has overwhelming odds in it's favour instead of the odds being overwhelmingly in the party's. Actually, thats the monster playing smart. The DM playing smart means grinding stupid monsters against the party in such a way that the party has fun. This means letting the defender defend, the controller control and the strikers kicking serious behind.