Lets also not forget that there are 3 spots for every 2 MDs trained in the US. There is a very large excess of NRMP spots for the near future. There are only about .7 AOA seats per DO trained. There is currently no reason why MDs can't rotate at hospitals with AOA rotations, there just isn't a lot of *sense* in it since they aren't eligible for the residency spots. I totally see the logic in opening all spots to everyone, or opening them to everyone who has qualified as sufficiently trained in osteopathic manipulation and diagnostic theory... but despite this, I gotta agree with NW here, there is some level of bias built into some schools. While we shouldn't go eye-for-an-eye, the sheer numbers argument of extreme excess vs not enough to cover everyone should keep the AOA match entirely DO for a long time still.
(though 20 years from now when demand for all spots is high, i think the combining will happen by then, if it didn't happen earlier)