At a certain point, the quality of compression will increase, and the bandwidth available will increase, to a point where the content being delivered via streaming is bit-for-bit identical to what's on the physical disc. There's nothing magical about the disc itself; it's just a container to...
I generally use iTunes and Vudu for rentals, but still make my purchases on physical media. There were a couple times where a movie I wanted to rent was maybe $4 for the rental or $5 to purchase, and in those cases, I've bought.
I'd also be open to buying an HD digital version of a title...