It's not a whole different mix. It's the same DD 5.1 that's already included on the Blu Ray Disc. They're just stripping off the lossless, just like they strip off the "extras". This is an obvious attempt to get people to buy the disc.
However, if people who really care about the "best possible" audio rent the disc and it sounds and measures blah, is this an incentive the buy the disc hoping that the audio is better?? I doubt it.
Of course 99% of the population couldn't care less and don't have the equipment to tell the difference anyway...