Valid concern, Mafia, and I should backtrack, the issue certainly isn't one confined to the occasional prick cop on the job. I would argue, though, that if the problem is as systematic as you've stated (and I have agreed to as a general statement -- I don't know if we're necessarily agreed on the degree of it, but as a general statement I do agree the problem is woefully systemic), they don't need another law to carry out prejudice and abuse. It seems as though the same result would happen regardless of this law being on the books or not. The negative consequence of the law that we've been ascribing to it is a negative consequence that already existed prior to the law's being enacted. And when the law IS justly executed, it does help. If we can agree on those two premises, then I would say the law has a net positive because it at least leaves open the door for being used justly -- and if it isn't used justly, the result is the same as it not existing, because the prejudice would exist anyway.
(And yes, it's a terrible statement to say "Well, the cops are gonna harass Mexicans anyway, might as well let them do something good if they end up harassing the right ones." I'll admit that those standards for law enforcement are abhorrent. But I won't pretend to have a solution to those standards -- only pretend to have a way to make something positive out of those standards.)