It takes more than one shot to completely disable an attacker, especially if you're only carrying a 9mm. Unfortunately movies have portrayed handguns as instant kill devices (laughably exaggerating the shot by pretending it would actually blast the opponent backward), but even those with great aim and calm in bad situations won't always aim well, and an enraged attacker can continue to be a threat even with several rounds rattling around inside them.
If you do have to walk through area where gang violence is a problem, you won't necessarily have the time to swap clips.
Further, it's not going to change things. Need 30+ rounds and can't carry them all in one clip? Carry three clips. Are they then going to restrict the number of bullets any one person can carry at any given time? Not to mention that those who would use weapons to attack others won't obey the law, and eventually they may be the only ones willing to pay the top dollar for the remaining extended magazines on the streets. Extended magazines, of course, which will now become a hot item, and be produced at an amazing rate until the law goes into effect, if it is passed.
All in all, it's a silly law - someone's attempt to remain relevant as a lawmaker.
