McCormick on Evidence points out that you should avoid referring to “the exclusionary rule” in the singular:
Discussions sometimes assume the existence of “the exclusionary rule,” suggesting that there is only one remedial requirement involved. This is unfortunate and misleading. Litigation and discussion is often dominated by considerations of the Supreme Court’s construction of the Fourth Amendment to the United States Constitution as requiring the exclusion in both state and federal criminal prosecutions of evidence tainted by a violation of that provision. But this ignores that exclusion may be required because evidence was obtained by violating other legal requirements, many of them not embodied in the federal constitution. Moreover, the contents of these exclusionary requirements need not necessarily be the same as that of the Fourth Amendment exclusionary requirement.
Generally, then, discussion best avoids simplistic reference to “the exclusionary rule” as a single rule covering a range of situations. Instead, this area should be conceptualized as containing numerous possible exclusionary rules or sanctions.
With that out of the way, McCormick then points out why the exclusionary rules might not be quite as effective as we might think in deterring illegal action by police officers and other government officials:
Deterrence consists of motivating persons to consciously choose not to violate legal requirements because of a desire to avoid rendering evidence inadmissible. Usually in exclusionary sanction debates this means encouraging law enforcement officers to comply with legal requirements in a conscious effort to assure the admissibility of the products of their investigative efforts. But detractors of the exclusionary sanction approach argue that any expectation that deterrence will work effectively is naïve, because law enforcement officers will often perceive the threat of exclusion as far less meaningful than other considerations influencing their conduct.
Exclusion will be a possibility only if the case is actively contested. Most criminal cases are not ultimately litigated, so the technical admissibility of evidence will not be a consideration. In the infrequent cases in which exclusion becomes a real possibility, the threat materializes only long after the officers’ role in the case is finished. A threat to exclude, made in the context of plea bargaining and protracted processing of criminal cases, may be a threat of such minimal and distant significance that it cannot be expected to overcome, in the officers’ mind, other considerations that suggest different courses of action.
In actuality, other considerations may be more immediately pressing and make stronger cases for officers’ attention. If an officer believes that compliance with legal requirements endangers his personal safety, he is unlikely to ignore that risk because of the possibility of legal challenges to the admissibility of the products of his actions at some distant time. Similarly, the expectations of the officer’s peers and immediate supervisors may well conflict with what the law requires and may compete quite effectively with evidentiary rules for the officer’s response.