Theoretically the answer is NO. Of course it sounds different if you have 10 loudspeakers playing simultaneously but if they playback the same material (the same mono signal) then it will be MONO, no matter how many loudspeakers you have. Well, the only thing that makes it as a so-called “pseudo-stereo” is the little delay that might emerge because of some loudspeakers are further away or closer to you than the other. You might compare this to the Cubase’s built-it MONO > STEREO effect which makes a Monologous signal as stereo by intelligently adding a small delay to the left and/or right channels. I mean the monologous signal can be thought as a stereo file with having the same material on the left & right channels. Of course the Mono signal only has one channel but your, mine, and everybodys mixers split that Mono signal into two separate signals and direct the to left & right channels.
So if you have a truly monologous stereo signal (the data in the left channel is exactly the same as the data in the right channel) there is no use of keeping that as a stereo file since your ears won’t hear the difference.
Yep, a bit off-topic here but wanted to point that out. You know, one of my supposed-to-be guitar guru friends said to me some 10 years ago that “if you record your guitar as a mono recording and then split the mono into stereo, then it is a different thing than a mono recording”. Yeah, sure it’s different; it’s double in size but that’s it all!
Oh, and to comment on the rest of your post, many bands today mix their material (especially Rock/Metal bands) so that the stereo separation is not that huge; I mean rhythm guitars are of course left and right, but the drums are pretty much in the center channel (except for maybe hihat and symbals/ride), vocals are (of course) middle, and basically all that doesn’t ABSOLUTELY have to affect the stereo image, is quite center-mixed. That’s because when listening through cell-phones (mono usually) or distant sound sources (the stereo image becomes “flattened”), it basically sounds better. So more important thing for a mix is that it sounds also good in Mono. Of course in Stereo too, but if the stereo mix won’t sound good as a mono, it is bad.
If everyone had a stereo headphones, then we wouldn’t have to worry about this at all But always check your mix also as Mono. It’s far more easier to pin-point certain flaws in your mix that way.