I agree that it shouldn't, but it did on Blip.tv. For the benefit of others, Kenny and I believe we just determined that Blip only offers a quality 4 stream. Lowering my bitrate in that case caused the Roku to buffer longer and not rebuffer on my slow connection. However...
As Kenny suuests, this works differently on Netflix, presumably because they run 4 streams. I ran a quick test by upping the bitrate to 3.5Mbs, and I do get a quality of 4 whereas I normally get a 2. All hail Kenny.