"but you need to preview the individual tracks"
On preview level...
That is absolutely true. The issue is that...mixed demos are only wma and to play whole midi style, you need built in GM MIDI synth embedded for previews (That is always there, regardless of "default plugin") otherwise with so many different setups it would create havoc.

My main issue with whole midi style / individual track situation is that... It is creative content that is very usable, but if it doesn't work right or intuitively, it is simply lost for most people, with exception of using whole styles "as is".