The information still needs to be added, from scratch, to begin with. As in, this isn't some "slap it in at the last minute and hope it turns out okay" feature. That's the sort of thinking that leads to things like the Great Oblivion AI Fiasco. If the information were "just made a default part of" everything near the beginning of the 5-year-ish development cycle that's one thing, see?
Agreed, there is a lot more work that needs to be done than just adding a climbing marker to buildings. There is animations, sounds, getting the AI to react properly, literally dozens of things that need to be done. Too late for it this time.
The only time I can think of something being slapped on at the end of development and not svcking was Goldeneye 007's multiplayer. About 2 weeks before the game went gold Nintendo asked "Can you guys add some functionality for all 4 controllers for multiplayer?" The devs went to two of the programmers, stuck them in a room with the code and said "Multiplayer, make it work." It did work out pretty wonderfully though. Paving the way for all of these stupid generic shooters that are only made because they have online MP.