Snapshot saves a small amount of time aiming a weapon - less time transitioning to looking down the scope. In that sense, it could be helpful - in some situations, fractions of seconds help. The main use of the perk, though, is quick scoping. Aiming down sights gives a significant bonus to accuracy, making headshots easier. Snapshot allows a rapid aim-fire button tap to get this bonus. In this sense, it doesn't help rocket launchers much.