you'd be amazed. People will steal a whole paddock-load of them and take them down to the market, they can make a few hundred easily that way. Generally we paint them with outdoors paint or use huge permanant markers. My rugs are old and tattered now so it's unlikely anyone would bother. But yeah, at my old paddocks we'd get cleaned out at the start of Autumn without fail. Only Desi, Dan and an ancient old pony would be left alone.