It bears mentioning in more detail:
If wizards (and other casters) can be disarmed of their wands (etc.), but they can still cast fireballs and flame strikes, they are hardly defenseless. Even if those spells do less damage, they are still capable of doing damage.
But, deprive a fighter or rogue or ranger or paladin or warlord of his weapons, how much damage are they going to do?
Really, imagine a dragon in his lair with some powerful trap at the entrance that makes all adventurers lose all their gear when the enter - it teleports all the weapons, armor, wands, etc. to the dragon horde and leaves the adventurers standing there naked. Now the foolhardy adventurers decide to press on and fight the dragon to get back their gear. Who is that dragon more afraid of? The naked fighter, rogue, ranger, cleric, or wizard?
I don't imagine Mr. Naked Fighter will grapple the dragon to death, nor is he likely to punch it to death either. Mr. Naked Ranger is picking up baseball sized rocks from the lair floor and hurling them at the dragon. But Mr. Naked Wizard is blasting the dragon with slightly weakened disintegrates. Who is the bigger threat?
Maybe spellcasters should be hosed without their implements. Non-spellcasters most certainly will be.