I used to have a stereo setup integrated with a HT setup, ie just add centre and surrounds and a AVR to drive the centre and surrounds. Later moved on to separate stereo and HT setups. Stereo system being a lot more musical and involving, than the HT.
Until I tried out SACD/DVDA and added a Yamaha AVR. Then the multichannel setup began to have something worth listening for, even in stereo! Of course diff rooms meant that the room treatments were required to get the rooms to sound good in the first place. Then I added some treatment to the HT setup and now it sounds like a serious contender against my stereo setup, just a different perspective.
What I'm trying to say is that a multichannel setup can be used for both HT and stereo, provided that :
a) the room is properly treated
b) things which are added for HT, eg TV, centre spk, sub, etc... are not too intrusive acoustically. I'm using a pull down screen with a carpet in the centre, so that helps.
If you want to watch a movie properly, then a serious multichannel setup IS required. There is no alternative. There are lots of information encoded into DVDs now, if you count the ES/EX, up to 7.1. Forcing that into 2ch stereo setup just doesn't do justice to the movie.
A lot of the problems associated with bad sound from DVDs are really created by owners who have not taken as much care with their multichannel setups, as they have with their stereo setups, so any comparison would obviously favor the stereo setup. Eg having the same brand and range of spks across all channels, as well as the amps, and not forgetting room treatments, and cabling, etc...
So IMHO, watching movies in stereo is just scratching the surface. The only exception I can think of is if you are using top flight spks which can throw holographic images and they must be full range to 20hz, else you're just kidding yourself. Watching the movie in the cinema would be more satisfying.