You used fungible. That was eating at me so I looked it up. You state what I have stated, unless I am mistaken. A fighter can use his abilities to defend. Or are you saying he is a defender because he uses his abilitie to fight enemies.
What I meant was that anyone could step in and do what a fighter could do, and other resources could, likewise be used to make up for the loss of a fighter. Maybe not as well, but in a pinch. For instance, if you lose your rogue, the Wizard can use Knock and the Cleric Find Traps, but there are still a few rogue skills left un-covered. OTOH, if you lose the fighter, anyone can take up space and do damage in melee - including a charmed ogre, animals the Druid has made Friends with, 0-level mercenary hirelings, Summoned Monsters, the expensive Golem you made, etc, as well as members of any class to varying degrees.
All of those classes and resources - apart from the 0-level hirelings - are also usable to do things the fighter can't. So the fighter has always been 'the defender' because he was the most readily expendable PC. So, not because he /can/ defend, or because he chooses to defend, but because he's not really good for much else.
For that matter, in spite of its vaunted balance, that formula remains true in 4e. The defender's role is simply to take the hits for the less expendable members of his team. It's just a much more engaging and enjoyable role - for the player - than it used to be.
I suspect it's still a bit tough on the fighter, though...