Digital watermarking is a slippery slope. They certainly have the ability to make it visible, for example, when you try to print the ebook, or once a book has been transferred to more than 10 devices, or under any other arbitrary circumstances. And 99% of users will not read the terms to know when this stuff will kick in.
I don't upload my purchased stuff to torrent sites and still much prefer not having watermarks in it, and if there was software available to "cleanse" it, I wouldn't think twice about running the files through it. Same goes for allowing Windows Media Player to tag your own CD rips with license information it finds on the Internet--it's just not something you want to do.
Watermarking is just a more passive-aggressive version of DRM, and there's a large group of people who will always fight DRM no matter what.