It's fair because it's a world that's there. And the way people talk about KH1's worlds, you would sure think that they were talking about all of the worlds.
But really, the only thing that even made them remotely attached to the main plot was just that princesses were being stolen from their worlds. If it weren't for the fact that Alice was stolen at the end of Wonderland, that world would be no different from how Enchanted Dominion was for BBS. Princess in trouble, but saved in the end.
Obviously, princesses can't get stolen every game, otherwise -that- would just come off as bad writing. What they did in BBS was actually probably a much better way to go about doing the worlds in the future of the series (though, if they weren't split up into three they would probably be more impactful). in BBS the worlds moved the characters. That's what we should be looking for in the future. The problem with KH2 was that Sora went to these worlds, and he didn't learn anything. The worlds never moved him in any sort of way. They were one standalone experience to the next. The issue wasn't that they weren't connected to the main plot because hijinks weren't going down or princesses weren't getting stolen, it's that they didn't propel the main plot (being Sora's journey). If the worlds of KHII had actually moved Sora's character forward, in ways that seemed logical, I doubt people would have had as much of a problem with them.
Did Monstro really move the main plot forward all that much? All that really happened was that a key character showed up and messed some things up. But, how is that different from Vanitas showing up in, say, Aqua's story? They're very similar, but, for some reason, people seem to hold KH1 in a much higher light. Perhaps it's nostalgia, but, BBS' worlds function much closer to KH1's than the past games.