It will stop them being stolen for spares, as the chop shops will no longer take them in. The police are now getting 87% conviction rate on chop shops found with Identifiable smart water registered stolen parts. It's like leaving DNA at a crime scene lol!
If a thief nicks one he's unable to unload it, even just displaying the stickers is often enough to curtail his interest and make him move on. Of course it doesn't prevent joy riders, but that is a minor consideration for most owners. I will be setting about marking mine tomorrow as the kit is already registered to the car. The stickers have been on for a while, next to the Catloc ones. Knowing what I know now, if the PCSO had not gifted me one I would have gone out and bought one myself @ under£60. I'm sure it will be an insurance premium positive, eventually.