Well, that “downside” is the entire point. Czech publishers call this “social DRM”, which conveys the point well: if the book says it was printed for me, visibl

Well, that “downside” is the entire point. Czech publishers call this “social DRM”, which conveys the point well: if the book says it was printed for me, visibly, I won’t be inclined to distribute it illegally. It isn’t about enforcement (see sibling comment). And yes, you can strip visible watermarks, but this still discourages casual pirates well.