Davis v United States (2011) USSC No 09-11328, 16 June 2011 emphasises the rationale for declining to exclude evidence obtained by unreasonable search that had been established in Herring v United States (discussed here 15 January 2009). This is that evidence should be excluded if doing so would have a deterrent effect against police misconduct. At the time of the search in Davis, the search was lawful, but afterwards the law was changed by Arizona v Gant (noted here 22 April 2009). That meant that there was, at the time of the search in Davis, nothing to deter, and the evidence was admissible notwithstanding the later change in the law.
What this sort of deterrence aims at is police conduct that is in flagrant, deliberate or reckless disregard for the Fourth Amendment rights. Where the police act in good faith, or where they are merely negligent, and they violate the defendant's rights, deterrence is not called for and exclusion of the evidence would not "pay its way".
A more rights-centred approach to exclusion of improperly obtained evidence pertains elsewhere, but you knew that.
