Its not just about propellors, its also to help airport staff spot infringements of the manouvering areas and runways. Believe it or not, a fair proportion of runway infringements are caused by pilots walking where they shouldn't.. having a hi-viz on gives ATC and other airfield personnel a chance to prevent this sort of thing happening, before a nasty happens.
If you had to insure an airfield post 911, you would make sure everyone possible was wearing one, just to keep the premium down!!