I know a local developer who was thinking that by adding a watermark to his mapping he could stop spreading the image file in internet. I don't know how and what. Never been interested. I think the topic and the idea is simmilar - adding a watermark then if the customer releases the personalised image he can very easily be found and sanctions be tooken e.g. cut from map updates.
He succedded in way - the mapping is not spread in internet or...very very occasionally. But this doesn't help him won the battle - more customers. On the opposite.