"Why 33 rounds makes sense in a defensive weapon"

A discussion of why longer clips are useful is available from the Washington Post here.

. . . In fact, the extended magazine actually vitiates the pistol's usefulness as a weapon for most needs, legitimate or illegitimate. The magazine destroys the pistol's essence; it is no longer concealable. Loughner allegedly wrapped the clumsy package in a coat for a short distance, but he could not have worn it in a belt or concealed it for an extended period. It had really ceased to be a pistol.

That's why extended magazines are rarely featured in crime - and that awkwardness spells out the magazine's primary legitimate usage. It may have some utility for competitive shooting by cutting down on reloading time, or for tactical police officers on raids, but for those who are not hard-core gun folks it's an ideal solution for home defense, which is probably why hundreds of thousands of Glocks have been sold in this country.

Particularly in rural Arizona, given the upsurge in border violence, it's likely that residents feel the need to defend themselves against drug predators, coyote gunmen or others. Yes, they can use semiautomatic rifles and shotguns, protected by the Second Amendment and unlikely to be banned by local law, but women generally don't care to put in the training needed to master them. Nor can the elderly handle them adeptly.

For them, the Glock with a 33-round magazine is the weapon of maximum utility. You can load it on Sunday and shoot it all month. (Nobody wants to reload a gun while being shot at.) It's light and easy to control. You don't have to carry it or conceal it; it's under the bed or in the drawer until needed. When the question arises of who needs an extended magazine, the answer is: the most defenseless of the defenseless. . . .