[buwaya]

How big of a building could you climb with a grappling hook?
How long will it take, and what sort of target would the guys be climbing slowly up there?

[GingisK]

I can't think of a single instance where a really tall building would need to be scaled from the outside in a combat operation. That aside, you can use a grapnel on very tall cliffs by just shooting it from a mortar or using a rocket assist. Tall buildings are best scaled using cruise missiles at the base.

Modern tactics generally don't need this sort of activity.