Giving up a grenade for a grapple that goes on Cooldown seems like a hindrance instead of the fun it implies.
Either make the Grapple infinite or make the Cooldown very minimal. It was advertised that you would be swinging around Destiny with the Grapple and speed hitherto unseen.
-
The grapple should be infinite/have a hella fast cooldown, but the melee-nade you can do with it should have its own, separate cooldown mechanic within the ability. That would result in unlimited swinging around with the occasional grapple melee-nade.