I have looked closely at the A7r for that reason but there seem to be three big question marks - light leak, AF performance and IQ when using an EF adaptor.
The light leak story is imho overblown, thats unless you regularly shoot with dense ND filters. Mileage varies with lens choice - how tight are the tolerances of the mount and how susceptible to stray light effects is the lens as such?
IQ-wise the adapter contains no optics, its just an extension tube. While the additional tolerances introduced by the second mount are measurable most of the time just about every other source of error will dominate.
AF? I got the A7r for its manual focusing facilities.
Esp with the TS-E lenses AF is...well...you'd be waiting for eternity. With native lenses its ok, with promising developments(see a6k), adapted its more a proof of concept to me.