I agree that for the most immersive movie experience, a good quality 5.1 (or more) setup is the way to go, and anything less is a compromise.
But, many want high quality stereo sound for listening to music too, this is fine if you've got lots of money as you can have it all, but if for example, one has a pair of Dynaudio Excite X16s and a powerful amp like a CA850, and wants to complete the set for movies, then you'd need a Dynaudio exite centre and rears (if matching is as important as some say), a good quality sub (perhaps Dynaudio) and a good quality AV receiver.
That's going to cost quite a considerable amount of money, probably more than double the initial outlay for the amp and speakers, and you're going to have to accomodate it all, more boxes, leads, cables, speakers etc.
So, while not having a 5.1 system is a compromise, it's one I expect many will make, as it means that if one is limited in what they can spend, they can have higher quality stereo sound whilst still having very good quality and enjoyable movie audio, with less clutter and boxes to boot. And that's a smarter compromise IMO.
I'll get the AVI sub one day though, movies will definitely be better with that