The best use for falling grapple is to beam-cancel on your 2nd bomb. By this, I mean if you are performing Samus' bomb-hop recovery, and want to throw them off, a good alternative to beam-cancelling on your top bomb is to falling grapple to the first bomb you dropped (the lower, 2nd bomb) in order to beam-cancel there, which throws a lot of people off. Make sure you can restrain yourself though, because if you do a perfect falling grapple you'll overshoot the bomb and die.
The 2nd best use for falling grapple is a desperate attempt to sweetspot the ledge in a hurry when you're way too high. You can still be intercepted, and now won't have your grapple, but your odds of making it are a little better than trying to just fast-fall there.
On a side note, if you air-dodge and would rather sacrifice invulnerability time to get down faster, just grapple and fast fall. You'll become vulnerable, but if they're far away, it's the best way to get back down and in control of your character again. Similarly, if you have to air-dodge onto the level, it's always fun to deploy your grapple at the last minute to try to stun them. This comes as a suprise to many foes.