The main reason why bluray movies have 5 (or more) channels of sound was to give people at home the chance to have high quality sound and vision the same (or better) than their local cinema.
Most (if not all) bluray players should have in the menu a way to downmix the 5.1 to stereo so if you only want to use two speakers then you should still get decent sound but not surround, for this you will need a a/v amp and three other speakers.
To answer your last question, it would cost more when mastering the discs to put more than one type of soundtrack on them so most of the main films dont bother, I do however have a few bluray films from independent film companys that have more than one surround format to chose from.
hope this helps.
*music2*