Here's a question, if a warrant were required to use a Stingray (it is, in some places) and if the Stringray intercepts everyone's calls, shouldn't the warrant have to cover all of the phones that would be affected? And if that's the case, how could you possibly justify searching the communications of someone you know isn't a suspect?
Imagine if the police said they wanted a copy of every bank statement issued by Bank of America in the last ten years, because they had evidence that the proceeds of a robbery had moved through that bank. Now imagine they wouldn't tell you how long the irrelevant information would be kept or what they would do with it. Of course, in reality, the police don't tell judges that they're using a Stringray, and they certainly don't tell them what extraneous information they'll pick up. They also don't obtain a warrant, but they do get a court order. I wonder if there's even a prohibition against the police department selling the information they collect to a third party.
Unless there are very strict rules about what the police can use the information they captured for and how long they can keep that information, yes, I'd rather let the robber get away. It's only money.