Well it keeps the filesize lower, but more importantly, it's a complete waste of time to fill in areas no one will ever see when playing normally. Some people actually make eyecandy in those spots. It's much better not to devote effort to them. I usually leave them empty or use solid tiles that are copied from another part of the level.

No problem if spectators can view that part though. Lots of 3d games let you clip through walls and even outside the level when spectating.