As ps1082 pointed out, the Buena Vista name is far from worthless, but Disney, ABC and ESPN are much more recognized brand names. I suspect that outside the entertainment industry and Disney geekdom, not a whole lot of people know what Buena Vista is and couldn't care less if Disney changed the name.
I agree that "Disney" is more than just a brand . . . to us. The reality is, however, that the average Joe on the street sees Disney as just a brand--a brand that is about to have (at least) one more component identified with it, broadening the name recognition even more.