Whatever the job, it's natural enough to want to take the easy way out. So historically, law enforcement officers have tried to find an exception to the warrant requirement whenever possible, to avoid the time-consuming and often unfamiliar process of applying for and serving a search warrant.
There's nothing wrong with warrantless searches conducted under such recognized exceptions as consent, incident to arrest, "fleeting targets" (probable cause vehicle searches), officer safety, exigent circumstances, and probation or parole conditions. And when a warrantless search is justified by a standard exception, you're not required to get a warrant, just because you could. Such a search "is reasonable and thus consistent with the Fourth Amendment, irrespective of the availability of a warrant." (Fernandez v. California)
But there are good reasons why officers need to become more comfortable with writing search warrant applications, and to delay non-emergency searches until warrants can be obtained.
Warrantless Searches Are Presumed Unreasonable
"Searches conducted outside the judicial process, without prior approval of judge or magistrate, are per se unreasonable under the Fourth Amendment." (Katz v. U.S.) This means whenever you rely on an exception instead of getting a warrant, your prosecutor will have the burden of convincing the court not to suppress resulting evidence.
On the other hand, when you search under a warrant, the defense will have the burden of showing some substantial defect in the warrant, or some mistake in your service of it, before the court could suppress the evidence. This is because the courts have expressed a "preference for police action taken under a warrant as against searches and seizures without one," and have said that "a search under a warrant may be sustainable where one without it would fail." (U.S. v. Ventresca)
Also, even when a Fourth Amendment error occurs, the "good faith" doctrine may save evidence collected under a warrant, while an erroneous warrantless search would generally mean suppression of the evidence, notwithstanding the officer's good faith. "A warrant issued by a magistrate normally suffices to establish that a law enforcement officer has acted in good faith in conducting the search. When an officer acting with objective good faith has obtained a search warrant from a judge or magistrate and acted within its scope, there is no police illegality," and the evidence is therefore not subject to the exclusionary rule. (U.S. v. Leon)
Warrants Provide Greater Protection Against Civil Liability
When you and your agency are being sued in federal court for civil rights damages based on alleged violations of the plaintiff's Fourth Amendment rights, you're entitled to "qualified immunity" from suit if your actions, though unjustifiable, did not violate clearly-established law.
The Supreme Court has said that "Whether an official protected by qualified immunity may be held personally liable for an allegedly unlawful action generally turns on the objective legal reasonableness of the action. Where the alleged Fourth Amendment violation involves a search or seizure pursuant to a warrant, the fact that a neutral magistrate has issued a warrant is the clearest indication that the officers acted in an objectively reasonable manner." (Messerschmidt v. Millender)
And as the Ninth Circuit Court of Appeals summarized the rule, "Under Messerschmidt, approval of a search warrant by superiors, prosecutors and a judge almost guarantees the honest police officer's claim to qualified immunity." (Armstrong v. Asselin)
Detention of Occupants
When you enter and search a house under a search warrant exception (such as consent or probation terms, for example), you don't necessarily have the right to detain everyone who happens to be inside. One of the occupants could simply walk out the door, concealing the very contraband or evidence you were hoping to find.
With a search warrant, however, you get the right to detain everyone present—both residents and non-residents—until the search has been completed. (Michigan v. Summers) Summers and other Supreme Court rulings "have permitted officers executing a search warrant to detain the occupants of the premises while a proper search is conducted." (Bailey v. U.S.)
Warrants Eliminate the Risks of Guesswork
When you act on your own, without getting a search warrant, you take the chance that a reviewing court may disagree with your conclusion that you had a recognized exception for a warrantless search. This could mean suppression of evidence and potential civil liability, whereas as discussed above, a warrant would make such outcomes unlikely. "In choosing to search without a warrant on their own assessment of [exceptional justification], police officers lose the protection [of the good-faith doctrine and "qualified immunity"] that a warrant would provide to them," if a court rules that the exception does not apply. (U.S. v. Ross)
Search Warrants Increasingly Required
The recent trend of U.S. Supreme Court rulings has been to restrict some standard exceptions and require warrants where they previously had not been necessary, including in the following cases.