Hiding in ink on the wall does present some nice opportunities though. I fought against a roller that would hide in the walls on the narrow upper paths on urchin underpass and hit me as I went by. I eventually just painted all the walls to stop that, but it sure caught me off guard the first few times. People are so focused on painting the ground they won't even bother with your hiding spot on the wall. On maps like blackbelly skatepark and port mackarel, there are spots where you can't paint the floor, but you can paint the walls next to you. Painting the walls there is necessary if you actually want to camp in those spots. Aside from all of that, I can't really think of any reason to paint walls beyond simply needing to climb up.