They only work in CQC (close-quarter-combat) and are fairly unreliable. A normal grenade to the feet and your enemy is gone, be it in midair or in a direct confrontation. Normal nades can be used for boosting in combination with colliders, to push allies etc.
I usually see cluster grenades on zombie servers, because they stand for extreme madness when the earth is filled with corpses, so it is hard to miss.