The problem is that you can't please everyone. On the one hand, they could play all the "classics" (Pink Floyd, Dire Straits, Paul Simon, The Eagles etc) which is good as it's a handy reference point for many people. But then again, many people would be saying, "they're all playing the same old crap again". The last thing most of us want to hear is Hotel California. Then again, if a system produces something like nothing you've ever heard before, it really doesn't matter what you're listening to.
Then again, they could play some more obscure artists/tracks which can sound stunning, and it will show off what a system is capable of, but it's usually the stuff people have never heard of, so there's no point of reference.
I welcome the diverse music choices that manufacturers make to showcase their products at shows. Sometimes this can be a fine art, and really shows whether some of these manufacturers/reps actually know their music or not, and know what to use to really make them shine, particularly in the confines of the less than ideal hotel rooms. I also think that this hobby is more about sharing music than analysing it, and there's nothing wrong in hearing something different/new. Thanks to AudioQuest, I now have The Dead Weather in my CD/record collection, to give just one example.
I think you should be allowed to take your own music along to audition everything, and a manufacturer should be confident enough to play absolutely anything on the systems they claim to be so good (although there are some albums that will just sound naff full stop). Give me a room for the day and I'd be playing all sorts of weird and wonderful stuff that would be guaranteed to impress.