Basically, non-Tome of Battle melee goes like this: you either grab a big stick with both hands and Power Attack for loads, or grab a long stick (which are basically all two-handed weapons) and knock people over with Improved Trip and friends. Using a hand on a shield means that you can't wield your big or long stick. And what do you get for it?

You're probably using a heavy shield, so +2 to AC. Later maybe you'll have the cash to spare on some cool enchantments for it, but most of the shield-only stuff is terrible, so all you're doing is saving money by distributing enchantments between your armour and shield.

The thing is, there's no reason not to have sword & board guys in the campaign world. A level 1 warrior (as in, most of the soldiers in all of our history) benefits greatly from a heavy shield. He's probably wearing medium armour and has a handful of Dexterity, so his AC is around 15. A guy exactly like himself probably has about +4 to hit (+1 BAB, +2 Strength, +1 Weapon Focus), so he'll hit him half the time. A heavy shield drops that chance to 40%. On the flip side, they're swinging weapons for 1d8+2. If they dropped their shield and grabbed the weapon in two hands, they'd get...1d8+3. It's not like they're going to get a different weapon from their quartermaster just because they decided to be uppity about it.

Part of what makes a warrior look heroic is ditching their shield for a bigger stick. Why do you think so few main characters who are part of a uniformed army actually wear helmets? Because it makes them look like badasses who tank damage with muscles.