I totally support the idea of a grapple hook.
I always thought of it as a reusable hook, you launch, it comes back to you and you can launch it again...
BTW, what happens when two people both hook onto the flag carrier? And from opposite directions?
Simple, your mate hooks on as well and it turns into a Tribes based 'tug-o-war'. That would look hella funny.
The hook would also need limited distance, unlimited distance is stupid. I can just see a route where the capper stuffs up with the grapple hook. Accidentally shooting below him and then 3 seconds later getting the whole whiplash effect (thanks to ragdoll physics).
This sorta reminds me of the teleporter in UT. I'm pretty sure other people have tried this, but you shoot the teleporter disc thingo and then teleport and you repeat that. Except you keep shooting into the air. After a while you could get pretty high to reach those hard to get to places.
And as R@NDOM said earlier, a map where the grapple is a key element, the whole spiderman cities, would 0wn!