Secondaries in this game are all pointless really. Usually in a prestige I get about 50 secondary kills. All the pistols are rubbish with the exception on the python, which needs speed reloader and SoH to be useable. All the launchers suck and the specials are only there for pissing about really. I will always switch out my secondary for a gun on the floor as soon as possible, especially in game modes like SnD.
Not useless at all. Compared to MW2, they are obviously underwhelming but they have their uses no doubt. Though, I do agree that specials are mostly there for "fun."
My choice for secondary depends on my primary weapon and game mode.
In Dom, I'll usually use a launcher— RPG against groups and flags. Law is great for all-around, and taking out annoying counter spy planes.
Most other game modes, I'll use a pistol.
The ASP is surprisingly great coupled with those guns that tend to eat ammo. It has the quickest draw and highest potential RPM. The ammo count isn't there, but you shouldn't be relying on it anyways— it's clearly for those times when you just need 1-2 more shots.
The CZ is better with snipers, semis, and burst rifles. Slap on extended mags and you have a reasonable backup. I avoid the full-auto because it actually slows potential RPM.