This is a matter of some debate. The general belief is that the exclusionary rule harms law enforcement because it "handcuffs" law enforcement by not allowing them to use all possible effective methods. Because of this, it allows guilty criminals to go free.
Under the exclusionary rule, any evidence that is judged to have been found illegally may not be used against a defendant. This means, for example, that a gun found in an illegal search may not be used in evidence even if the police know it is the murder weapon. When this sort of thing happens, it lets guilty people go free simply because the police made a mistake in procedure.
Some argue that the exclusionary rule actually helps police, though, because it forces them to be meticulous and careful in their investigations. This side says it makes them better, more professional law enforcement personnel.