> When you buy a book you also own the book, and not the intellectual property it contains.

But a book is not a movie. You can't take your physical book and let 500 people read it simultaneously. So there's no need for even considering restrictions on what you can do with your physical book. The closest thing that comes to mind is scanning and re-printing the book 500 times and distributing that. And you can't do that. That's illegal.

> I don't see how it correlates with any necessity for DRM.

Because of the extreme ease of reproduction + distribution, coupled with the near zero chance of being caught, is a strong incentive to violate IP law.

> Limited time of rental is justified for physical goods. For digital it's not (I explained above why).

No you didn't. What you said has absolutely no bearing whatsoever on the validity of limited-time licensing of the right to view content.

> I don't see any sense in limiting access. Netflix can charge the same thing for unlimited access, plus allowing making backups and still make the profit

Hah, in your dreams. Netflix's ability to negotiate (with the content providers) for the legal right to stream content to you is predicated on there being limitations. The more unrestricted the access, the more they have to pay.

Not only that, but if you did remove all limitations (and restricting access to the duration of your membership is, of course, a limitation), then there would be no reason for people to stay Netflix members for longer than it takes to download and save all the movies they want to watch for the next few months. Subscribe for one month, download 6 months worth of movies, unsubscribe. Doesn't sound like a great business model, does it? But of course they couldn't do this even if they wanted to, because no content provider is going to give Netflix a license to re-license the content to Netflix subscribers in perpetuity.

> No, DRM can't enforce it (since this stuff is pirated practically instantaneously). So why is it used?

Because it's better than nothing. New content is pirated pretty fast, sure, but stuff doesn't stay easily available forever. Netflix doesn't deal in new content anyway (barring their recent ventures into producing their own content, which is a different discussion).

Without DRM, if I have a friend with a Netflix subscription, I can just ask him to grab a few movies and send them to me. Not only does this work even if the movies aren't easily available on BitTorrent, but it's also a lot safer because nobody can track this (whereas BitTorrent does have a non-zero risk of being tracked, or of quality problems, or fakes, etc). At this point it's about convenience. Make it hard enough or risky enough to acquire the content for free, and people will decide it's easier to just pay for it. And that's what DRM does.

Granted, DRM solutions do not work particularly well. But in the eyes of the rights-holders, it works better than nothing.

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To clarify, I'm not an advocate of DRM. I would much rather get content without it. DRM-free content is more convenient to work with than DRM'd content. But there is a vast gulf between saying "I'd rather have DRM-free content" and saying "All content must be DRM-free". The former is something I can ask for. The latter is just wishful thinking.