Well, the smart assed answer would be the Secret Service, TX DPS, the Air Marshalls, and many of State Police forces ;-). They use them because of the enhanced ability to penetrate and tap barricaded suspects. I owned an SAXD 4" chambered in that calibre and while I enjoyed shooting it, it did have a helluva punch to it, and I would not feel comfortable carrying it for defensive purposes. Waay to much possibility of over penetration (which is why I can't understand the Air Marshalls using it). This round, as has been stated is expensive, and it is not yet widely available. I prefer to keep my weapons in calibres that I can find anywhere, any time (.45ACP, 9mm, .223, 12ga.) the only exception being my .338. If the shit ever hits the fan, I want to be able to pick up a magazine off of whomever is laying nearest to me, slam it home in my weapon, and drive on. Not a real likelihood with .357Sig.
drew