Don't be facetious, just because the Constitution says "don't do X" doesn't mean X is worthless.
There is value to freedom of association, but it shouldn't be assumed that it's valuable because the Constitution says so. The same logic can be applied reversely -- there is value to curtailing freedom of association.
"It's only illegal if you get caught" seems to be the mantra of the DoJ in this case -- stonewall and distract rather than address the legality of illegal activities.
This country was founded on open courts and due process, neither of which this "useful tool" falls under. My point is you can't "save the village by destroying it," not that things derive their value from their legal status.
The wonderful thing we mostly have going in the USA is destroyed by these individuals who pretend to be protecting it. The ends do not justify the means.
I'm not at all saying the ends justify the means, just that their argument that these tools are useful is valid, and we will need to help them realize that they are damaging us far more than they are helping us.
So far, nobody has really said what value comes from the (presumably illegal, and hence cover-up required) use of IMSI catchers. We've all speculated ("easier to find criminals!", "cheaper to find criminals!"), but what, exactly, is the value to law enforcement? The citizens of the USA are supposed to decide (through their representatives, of course) about what to do and how to do it. Perhaps the tools aren't useful, perhaps they are. Without some knowledge we don't know. The feds need to open up about IMSI catchers.