Well, at low levels, the fighter is great. But, that usefulness drops fast as he progresses. Especially if he's a shield fighter.
At low levels, he can deal a fair amount of damage, each round. Our goliath fighter in STAP would regularly deal around 20 dmg each round at 3rd or 4th level, while even the rogue landing a sneak attack critical might not get that close.
That, combined with their high AC, and things are great. The other fighter in our group (who was also a dragon shaman) couldn't be hit by pretty much anything - he was a TANK.
The problem with it all is that those advantages of the fighter drop out around 10th level or so. That insane damage turns into just a fair amount, which is often surpassed by mages who deal large damage drops. Also, fighters are frontline fighters, and against monsters that can easily deal 50+ points of damage, fighters quickly turn into "Do I fight this round and risk being knocked from 40 HP to dead, or do I hide and heal?" - they are almost a liability to clerics (and they're one of the reasons Clerics can be annoying to play - our cleric at 10th level really hated the fact that he had all these great options, but instead had to heal a fighter that couldn't even deal the same amount of damage!)
And that AC boost? Well, at 10th level, most any monster can hit you, regardless of your AC. Our dwarf with the AC of 30 was still getting hit quite often at 10th level, which isn't unusual. By 10th level, the mages are harder to hit than fighters - our mages would fly out of reach and shoot. Hell, even at 3rd level, mages can be nearly impossible to hit, with spells like Blink. The last mage I played, a 4th level Wu Jen in an Eberron game, made it a point to go invisible and then cast buffs and summon spells. Nearly impossible to hit at that level.
Anyways. In our STAP game, most everyone started out as a fighter - but by the time we got to the 6th episode, all of our fighters had swapped out their characters - one became a phanaton warmage (and later a duskblade, when the phanaton was killed by some poisoned arrows) while the AC monkey dwarf decided to play a wild mage. Fighters really stop being fun by the time you hit 10th.