I had a quick look at the guys' Achievements for Batman Arkham City to see how much they played of it, and was shocked to discover that Ryan finished the story without ever unlocking the grapnel boost. Brad didn't either, but he barely played the game at all.
I really can't imagine playing thru the whole game without being able to grapnel boost around the city.
I'm making my way through the story now that things have slowed down and the game is growing on me a bit--mainly in the instances where it reminds me why Arkham Asylum was so great--but I still don't think it justifies its open world very well.
As for the grapnel, you're probably looking at my PC achievements--I got that thing almost immediately on the Xbox when I started playing it around the time it came out. The way that thing works is one of my big complaints with the open-world traversal, actually. You can't see what you're grappling to mid-flight because you can't control the camera while you're holding down the A button to maintain your glide, but your flight direction changes based on the surface you grapple to, so you can't smoothly maintain your direction as you soar across the city. It just feels clumsy--and after playing stuff like Infamous and Just Cause 2 and Saints Row 3, if anyone should be able to get around an open world as nimbly as you can in those games, it's the goddamn Batman.