title_body: Is "soft DRM" a viable option? For "soft DRM" I mean a sort of watermark (for example, a section stating "This book has been bought by Jane Doe from ACME Inc") which does not protect the book from being copied but states who is the buyer.
  
  Yes, I know that deleting the watermark is a piece of cake: I am not concerned with this. I am just wondering if this makes sense or not, and I have mixed feelings, but I can't tell whether it is the fact that with a physical book this does not happen or not. What are pros and cons, in your opinion?
upvoted_answer: I don't know if there are studies into soft DRM, but there are studies
  on difference in behaviour between people that are anonymous and people that are not. The non-anonymous generally being more "honest".
  
  Removing soft DRM would rely on the non-anonymous person to do so, before handing the ebook on to someone else. Not doing so would have her/him run the risk of the passed on ebook being made even more public with name identification attached. I can only assume that for some of the potential passers-on, this is a deterrent. 
  
  You also would have to trust the software that removes the watermarking. Removing metadata is not enough, since the watermarking could be added via steganographic approach. Some formats such as PDF, have a plethora of possibilities to hide information. And you would have to remove images from an EPUB file, or severely change them, as there are watermarks for images that survive some of the re-renderings of the images. And your name could be watermarked in any or all of the images included in a book.