Radley Balko provides some useful background on Hudson v. Michigan, a case currently before the U.S. Supreme Court. If you're not familiar with Hudson, here's a quick precis:
The Hudson case the court is now considering deals with illegal no-knock raids. That is, raids in which police couldn't even manage to follow the almost-perfunctory hoops they're required to jump through to get a legitimate no-knock warrant.
In Hudson, police in Michigan knocked and announced themselves, but waited just 3 to 5 seconds before breaking into the home of Booker T. Hudson. Once inside, they found a substantial amount of cocaine and charged Hudson with various drug crimes. When a trial court found the wait time insufficient to satisfy the knock-and-announce requirement, Hudson moved to have the evidence suppressed.
The case eventually reached the Michigan Supreme Court, which ruled that suppressing the evidence seized in the raid wasn't a proper remedy for police violating the knock-and-announce rule, and cited the inevitable discovery doctrine: Because police had an otherwise valid search warrant, their failure to announce was inconsequential. They would have found the drugs anyway.
But the exclusionary rule's primary purpose is to serve as a deterrent against Fourth Amendment violations. If police know that breaking a particular Fourth Amendment protection will result in the suppression of any evidence they find, there's strong incentive for them to follow the law.
I'd actually be willing to give up the exclusionary rule, if it were replaced with real sanctions for police and other officials who violate suspects' Fourth Amendment rights. As long as that's a pipe dream, I'll take the rule as an imperfect substitute.