In general for ebooks, DRM exists to keep people from simply trading books around within their circle of friends, like people do with paperback books. As long as it discourages the casual piracy, i.e. sending a copy to all your friends so you can talk about it next Friday night at your book club, it serves it's purpose for most publishers and authors.
From a book seller point of view, as long as it keeps most people in the ecosystem, a la the Kindle and Books ebook stores, it serves it's purpose. There are enough old kindles out there than trying to put put some super DRM would be more trouble than it's worth.
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