Really? Have you actually seen players change their fighter's weaponry and styles around during play? Sure, the fighter might have a ranged weapon and a melee weapon, but, beyond that, I've never seen a play switch styles. If the fighter uses a shield, he always uses a shield. If he uses two weapons, he always uses two weapons. Again, barring specific circumstances when he switches to a bow. But, fighters have always baselined on whatever single style they chose at 1st level.
Maybe we've played different styles, but yes.
The fighter basically changes his main weapon whenever the party finds a new, more powerful magic weapon. Most fighters I've seen have a lance to use when mounted, regardless of their usual combat style. Having a tower shield for heavy duty defender action is also not uncommon, even though few would use that in all fights due to the -2 penalty to hit.
Sometimes the fighter borrows another character's weapon. The cleric's mace is more effective against skeletons and the cleric doesn't need it, since he's Turning Undead. The rogue's flaming sword works against trolls - and he'd rather stay ranged. Etc.