Smash is a unique fighting game for many reasons, not least of which is the effect stages have on the game. Unlike your typical SF or MK fighting game, the stage can dictate how effective certain play styles are. Some would argue that Dreamland benefits the aggressive, pressure heavy players, while Hyrule benefits the more patient, defensive players (please someone don't get angry and correct me, that part is an example and is so beside the point). If you change the ruleset to one stage metagame, you are giving the natural advantage to players whose play style strength matches the stage you choose. Personally, I don't think a ruleset should influence the metagame like that. In the current system, the first match is played on essentially neutral turf, and the ones that follow will be to either players strength, giving both a chance to utilize their best skill set.
If that isn't a good enough argument (and I doubt it is, I'm tired), just look at the other smash games. Both use a similar system that we currently use if I'm not mistaken. Now, obviously both of the other games have vastly superior stage selection, but I definitely don't think SSB stage selection is at the level where only one stage is playable. This is evidenced by two different communities, Peru and Japan (I know games a bit different blah blah not important here), playing on only Hyrule or only Dreamland, respectively.
Of course, if you make the system single stage then players would make their play styles match the stage as best they can. But to me their isn't a superior play style we should be supporting. And if there is one, who gets to say what it is?
And also.... nah jk I'm done writing for know. Haha I love being an engineering major, I've written more 'essays' on smashboards than I have in four years of college.