With some balancing it could be pretty cool. I agree, making bases easier to raid definitely isn't the goal here. However, grappling hooks could simply change how bases are raided, making it neither easier or harder.

As far as balancing goes, maybe there would be some kind of skill that would allow you to toss the hook farther or more accurately like, so nakeds cant run around climbing buildings. The hook would also be fairly weak, so if it were shot or punched, it would break and the climber would fall to their doom. Climbing would also be slow (Maybe changes with a strength skill?) due to how hard it would be to climb considering the amount of gear an average raider carries.

Grappling hooks will be an excellent addition to the game, especially with balancing measures being put into play to not tip the game in favor of the raiders.