part of my work is with subscription websites - porn sites, to be specific. what has universally proven true is that pirates find ways to steal the videos regardless of whether they use DRM or are embedded with no download option. the only people who can't save the videos are the people one would like to have save them - the legit members.
one of the reason ones would want the legit members to save the content is that later on, they tend to see it, say to themselves "oh, yeah - i liked that site!" and come back to rejoin. i would think that would apply to people who buy books from publishers and authors, too.
interesting to note, several large sites removed their DRM in the last year after using it. what they discovered is that they made more money in 2 ways: first, members rebilled more because they were happier to use the site. second, more expired members tended to return and re-subscibe, and they also did it more frequently.
the truth is that some people steal stuff - but it's not very likely that people who feel theft is a legimate thing were going to buy ebooks, other downloads or memberships anyway. as for people who are honest book lovers, they'll buy rather than pass around, partly because readers understand that it's a good idea to support the publisher and the author for karma and so they'll keep up a steady stream of new books