Have a look at the beginning of this video (I won't go into details about the room for tactical improvements).
The camera man needed 18 (!) seconds to ready his AG36 (M320) stand-alone grenade launcher (0:25-0:43) - and he was one of the few who were still functioning under stress at all !
I didn't see a better argument for under-barrel grenade launchers with other than iron sights yet. Likewise, this anecdote can be understood as an argument in favour of stand-alone grenade weapons that merit being treated as a primary weapon, such as a GM-94..