Antimagic field is the ultimate spell protection, but anything with any melee ability can just walk in and dice up your wizard, so its hardly the ultimate defense, not to mention once its in the field with you , you can't do anything to it.
Image Merlin casts his Antimagic field at one end of the battlefield.
Conan is at the other end, and drinks a potion of fly (or uses any other method of moving quickly thats available at level 13 or so), and moves closer (if not into the antimagic field).
Next round Merlin casts a single spell at someone (possibly Conan), assuming his target isn't taken out...
Conan moves into melee with Merlin. Merlin has an AC of 12 on a good day (assuming Dex 14 or so). Conan has a BAB +13, +6 Str (Raging), +1 MW weapon, so moves in and attacks Merlin repeatedly with a +8 Power Attack, or worse yet grapples Merlin and crushes him to death with his superior Str and 3 attacks a round.
Merlin's defense?
Drop his antimagic field and try to escape alive.
Its a good defense, but its VERY vulnerable to any character with more melee ability than the Wizard (which is just about everybody in a magicless environment).