A Grenadier will always (and should game wise) trump a soldier with a rifle grenade, that's fine. Consider rifle-grenades to be hand-grenades that you can launch very far with your rifle. They are not supposed to compete with 40mm HiLo Grenades.

France uses both the AC58 and the APAV40 and the SIMON has been adopted by US as well. The fact that a soldier can fire grenades without having to carry a launcher makes them attractive weight wise, especially if they double as handgrenades because e.g. M67 Frag weighs roughly the same as the APAV40. Potato masher style hand grenades can also be thrown further than standard hand grenades.
The AC58 on the other hand would also add some anti armor capabilities to the average rifleman.
As written before they add tactical flexibility.

@ Anachoretes: YES! A 9x19mm round produces less recoil than 7.62x51mm or 6.5x39mm because the casing contains less propellant