some VERY good points, most of which hadn't crossed my mind - in bold for tl;dr
http://gizmodo.com/5841675/why-netflix-just-cut-itself-in-halfIt's an obvious point, but it's the one that matters: DVDs are becoming obsolete. But there's no mass extinction event to point to; it's a lugubrious death, a slothlike shuffling off. Some people will be renting DVDs by mail for years and years after it makes any sense, just like some people still have Aol email accounts and buy full-size camcorders and watch Grey's Anatomy. So why not keep making money off those people for as long as you can?
Think of it like this: sometimes you need to amputate, and sometime's it's better to quarantine. The DVD industry has the sniffles now, and Netflix is putting it into a hyperbaric bubble before it devolves into something terminal—and brings down its strapping young streaming lad with it.
There are two main benefits to cordoning off the mail service in the Qwikster ghetto. First, it keeps streaming customers shielded from the inevitable DVD price increases that will come with fewer subscribers and ever-increasing postal rates. And—maybe more importantly—Netflix is able to limit the constant reminders of all the movies that you can't add to your Instant queue. Streaming customers won't be able to see DVD availabilities any more, which means you won't get that feeling that you're staring at an unattainable ocean of truffles and bonbons with nothing but a Werther's Original in your hand.
There's also this point raised by Bill Gurley today: Netflix has this whole time been paying studios for streaming rights based on its total number of streaming subscribers, including those with streaming + DVD plans that never watched a movie online. By partitioning streaming and mail-order, Netflix can hand studios a more accurate picture of how many people are downloading their content—which will hopefully mean more favorable studio deals, and more options for your queue.