Did the shooter take the phone with him when meeting with accomplices? If so, perhaps there's GPS info on the phone.
What are the gaps in standard behavior (i.e. phone turned off...). Are these times where the FBI should dig deeper to discover the actions of the shooter?
The idea that bulk collection of metadata is objectionable is something we're seeing in civilized technical circles. Individual collection of the same is even worse.
(What the probability is though is hard for me to predict as I don't have enough information, but assuming he didn't have any slip ups on his work phone is a bad assumption. If there is a 1 percent chance that future attacks of similar scope could be prevented by knowledge gathered about this case, is it not our obligation to find out? Its a complex issue that the courts and media will play out over the next few months)
No, it isn't. The phone they're trying to break into was the shooter's work phone, provided by his employer. He physically destroyed his personal phone before the acts in question. The idea that he kept relevant information on his work phone, which he left intact, is absurd.