After all, most of the cost of SAR is in the "search" phase.... and missing boaters (even MOB) usually die of exposure, not drowning. I think the 3-mile parameter is a bit tight -- going to include most boats other than dinghies and rowboats.

I don't really see a downside, other than the expense. But you can get a nice GPIRB or PLB for under $400 these days, and that's not a lot if you can afford a boat.