I think it would really add to the base building experience that's being developed if people could mark their own bases or their land around them.
Obviously the graffiti would have to only be able to last for so many hours and/or players could only graffiti so many surfaces (like if a player tags more than 4 spots the oldest ones disappear). Maybe players could only graffiti on the random billboards around the map. It would also cost 10% of the spray-can or something like that. Writing over another players tag could maybe replace theirs and that would be a new way of sticking it to other people in game.
Though this is somewhat like being able to leave notes, I feel this would be cooler because for as far as I can tell, no one leaves notes because no one reads the notes. If you can graffiti a surface and it lasts for some time, it will get seen since it will be much more obvious and in your face.
On a final note, I would love to see more color variants for spray-paint, especially if something like this gets implemented.