As far as primary pistol goes, I knew a few. VERY few. One of them just retired. I never got a good look at his revolver. He was LAPD. I would guess it was a 38 S&W, since LAPD did not have 357 authorized. If you see it, you'd think, "Wow, that thing has been around." Another one I knew carried a Stainless Steel S&W 38.
When I asked around in Los Angeles area, about 60~70% of the police academy graduates were buying revolvers as their back-up about 5~6 years ago. Others would choose a Glock 26 or something like it.
However, that seen some change recently. I see many of them dropping the 38 snubbies for 380ACP pistol.
I see popularity of snubbies dwindling down as a police backup, although I don't see it going away.
First reason is that many cops use the backup as off-duty weapon, and pistols like Glock 26 offer much more in that regard. Combine it with their duty weapon, then they even see a benefit in the ability to use 17 round Glock 17 magazine. Same with M&P and M&P Compact combo. For vast majority of cops, this is much more pragmatic approach. As long as they're proficient with a full size Glock or M&P, they can expect reasonable hit with their compact version too. With a revolver as a back-up, it would need more dedicated training.
As far as hiding it and comfort goes, pistols like Ruger LCP beats snubbies.
