Well, y'know, normally if the Fighter wanted to protect his fragile, bookish nerd-friend, he'd either be standing near the Wizard to protect him, or getting right up in the enemy's face so as to run interference. And the Wizard, who absolutely must have half a brain in order to cast any spells, would know better than to get close to enemies before he's ready to blast them to smithereens. Unless the group was ambushed, in which case, that's just the advantage of springing an ambush; you get to attack when the poor berks are completely unprepared.
Also, this is what polearms and spiked chains are for, of course. That's the only time you should be protecting your allies from such a poor position, because you have the reach to make it work.
Or, what readying an action is for. Enemy tries to move past you to the Wizard? Readied action and you move to intercept, getting in their way, so if they want to keep going around, they'll take an AoO from you because you just got up in their face.