IMO this is a great idea. Leonhard will be 32 in October but would sign for vet minimum. He's got a vast amount of NFL experience - and experience in the 3-4 although I don't think that matters as much at safety as it does at other positions. The only reason I can see for not to do this is because they signed a different UFA safety they liked better.
Right now the safeties on the team are Burnett, MD Jennings, Sean Richardson, Chris Banjo, and perhaps Micah Hyde if they are planning on moving him to safety. We've seen enough of Jennings, I hope they have too. I'd like to see a first, second or third round pick at safety if it makes sense at the time of the pick and Richardson has a chance to be a good safety. But what an inexpensive insurance policy Leonhard would provide. Leonhard's age is outside of Thompson's MO, but the cost would more than fit his UFA MO. And it would be a no risk signing: If Richardson and a rookie emerge at safety and/or Hyde and another player look to be the answer, they could waive Leonhard and it'd cost them nothing.
A cheap, experienced vet who will sign at vet minimum at a position of need who wants to be a Packer - c'mon Thompson, no reason not to do this (unless you sign a different UFA safety).