We know it's technically a bad weapon compared to others, but in situations were you get in a one v one fight it's annoying to get popped by a last second shot whilst the other play jumps about doing fuck all. It's just so skilless and cheap, makes me just think the player using it is a cheap noob (because they are usually).
Really it reminds me of things like the noob combo in Halo 2 (plasma pistol / BR) or USAS with frag rounds in BF3. Yeh it's in the game, but man it's cheap as fuck and OP.
The smart pistol takes so long to lock on. If you haven't killed the guy by the time it finishes, you probably deserve to die.