Assume that there actually is something interesting or useful on the phone. That still doesn‘t mean the FBI „needs“ the information. In basically any situation where crime has occurred, there is a ton of information that might be useful, but that is far from mandatory. Hell, in this case alone, there were the destroyed phones. It‘s much more likely that there would have been useful information on those phones. But no one‘s talking about how the FBI „needs“ that information, because everyone knows the FBI can‘t get it. And, since much of the planning for this attack must have happened between Farook and Malik in their home, the FBI is never going to know what they said to each other as they sat around the kitchen table, or on the sofa, or in bed.