This actually pisses me off, a lot. I don't put my ebooks up on torrenting sites or distribute them - but seeing my own email address show up on every page of my ebook means that I'm not going to buy from you again, ever.
To me, there's a distinctive difference between adding a non-audible watermark to a audio file vs. watermarking text with text. I can listen to purchased iTunes music all day long and never even be aware of the watermarking existing, but I can't look at a single page of my [one and only] Packt Publishing ebook without their giant logo, my name, date of purchase and my address being displayed in the footer.
If they wanted to watermark my info into the file to prevent file sharing, that's fine. Printed on every page? Obnoxious. It's another case of legitimate purchases having a disincentive that piracy doesn't. I bought the book, I know I bought the book, I don't need to be reminded 272 times in the process of consuming it.
I do - but having my email embedded into a part of the file that I'm not going to see while I'm regularly using the file isn't a problem. I'm not distributing, and I don't mind that the file's identified to me. Having my email on every single page of the ebook I'm reading is a waste of space that only serves to frustrate me - why is my email there? How does having my email there improve the reading experience? It doesn't.
It might not be as intrusive as annoying DRM, but it still makes reading my ebooks frustrating.
Yes it is probably a good deterrent for most people. The problem is you need something that'll work for everyone. Otherwise one person won't be deterret, will edit out the details, and put it on a torrent site. Then anyone who wants to pirate it, can pirate it.
I didn't know that about Apple, but I would imagine they are not the entity that encouraged such behavior. I'm glad I don't use iTunes at all, and now, I never will use it.
I think it's a perfectly good deterrent for most people without adding on annoying DRM or anything else that actually impedes the reading experience.
(By the way, you do know that Apple silently embeds your name and email in every DRM-free music file you buy from iTunes, right?)