I never understood 'the fighter is a defender' idea. And it is one of the top ten stupid ideas that 4E did: "Ok, your a fighter and by fighter we mean your a body guard''.
I must say that in most of my games I have never had a 'defending' fighter. In my game the fighter...well, fights. Shocking, I know. The fighter does not hang back by the spellcaster "Staz next tooz me spellerz, I'll protectez youz" the fighter is rushing into battle to fight and kill the enemy "Arrrgghh! Die monsters!"
I guess some people think the fighter is this: "Blagorn the mighty moves over to stand next to wizard Worn and a watch for enemies trying to attack him. No enemies? I just stand there"
But I think a fighter is: "Blagorn charges at the closest orc and beheads it with a swipe of his battle axe!''
The best defense is a good offense. Once the fighter kills the monsters, then the wizard is defended from the monsters....as the monsters are dead.