If the FBI hadn't released those photos on Thursday...
Would the MIT police officer still be alive?
Would the brothers still just be wandering around?
Would the FBI have eventually have caught them?
The Washington post article explains FBI thinking on why photo was released:
"●Investigators didn’t want to risk having news outlets put out the Tsarnaevs’ images first, which might have made them the object of a wave of popular sympathy for wrongly suspected people, as had happened with two high school runners from the Boston area whose photos were published on the front page of the New York Post under the headline “Bag Men.” At the news conference, FBI Special Agent in Charge Richard DesLauriers sternly asked the public to view only its pictures or risk creating “undue work for vital law enforcement resources.”
●Investigators were concerned that if they didn’t assert control over the release of the Tsarnaevs’ photos, their manhunt would become a chaotic free-for-all, with news media cars and helicopters, as well as online vigilante detectives, competing with police in the chase to find the suspects. By stressing that all information had to flow to 911 and official investigators, the FBI hoped to cut off that freelance sleuthing and attend to public safety even as they searched for the brothers."
In other words the NY Post cover, the CNN bungling the arrest, the Reddit super sleuths & endless speculation forced their hand.