Many of the measures in the absurdly-named "PATRIOT act" (the acronym for "SPITBOL" was less forced) are supposed to get law enforcement agencies more information. But you can have too much information --- and in some cases, they're already drowning in it. For instance, intelligence in Arabic is massively backlogged because of a shortage of translators.
And they've failed to make the best use of information they had, as in the case of Zacarias Moussasaoui, who got himself noticed at a Minnesota flight school in August by asking to learn to steer airliners in flight, but to save time and money by not learning the hard part of the job, takeoff and landing. That got him arrested --- but there was apparently little or no follow-up at other flight schools to ask if other foreign students had shown similarly skewed priorities. If there had been, who knows? Something might have turned up.
Would the FBI handle things any differently now? Not if every available agent gets detailed to ask immigrant Muslim housewives how much they know about Semtex.
You could make an argument for strict adherence to probable cause requirements, on the purely pragmatic grounds that the FBI's inevitably limited resources are best spent on information that is likely to actually matter...