Excellent points. The only reasonable response I can give (because I have one on order) is because "it's cool" and "it's new". For Canon users there's a more rational reason - the A7R gives them a high-resolution, high DR body that Canon currently doesn't provide, and with the Metabones adapter they'll get full aperture control, AF (albeit slow), and IS support as well.Which leads to the question - Why buy it?! Why buy something for 1700 or 2300 bucks that limits you so much, when there's perfectly good DSLRs that will do everything you want to do photographically with vast systems of optics that the A7/A7r will never match? (And no, I'm not counting kludges like adapters that don't transfer 100% functionality of the lenses.)
Only folks with big collections of orphaned lenses (Canon or Minolta manual focus, for example) have any real reason to consider these cameras beyond the "look how thin the body is" novelty, which is sure to wear off when you have to hold on to it with a front-heavy lens mounted to it. Why do you think all the new "native" mount lenses are slow in terms of aperture relative to focal length? Because the camera + lens will look pretty ridiculous as a package when you mount some high speed glass on it, that's why! Only suckers think that FF cameras (with lens, you know, how you actually use them) are going to be "small and light" because they're "mirrorless." The lenses don't shrink appreciably (if at all) for most focal lengths!
