EDIT; I just checked and the windup/winddown for the grenade assaults are the same as normal grenades throws... The delay seems to be on the ability side, which explains some.
However, the ready aim time is up to 2 seconds. Also it does seem the grenade does only half the damage across the board as a standard grenade.
So you have a grenade ability with up to 2 seconds delay before throwing animation even begins that hurls half damage grenades in a spread that cant be controlled.
It is good vs mgs in buildings and good for potentially wiping squads on retreat.
It is used against units like a standard grenade only due to its low cost and not due to its actual performance.
If one had to pick between a 20 munitions standard grenade or a 20 munitions grenade barrage, the standard grenade would be taken in a heart beat.