Billboards are an ugly smear on the face of the urban environment, whose sole purpose is to enrich the owners of the billboard, and those that rent them.
The difference between billboards and graffiti is that one is “OK” and the other is not. But in neither case do we get a real choice.
I don’t think billboards have much political legitimacy at all, but there is lots of money behind them, so they’re probably here to stay.