That's not what happens with beat'em ups, or Mystara in particular. Yes, it's fun to play with others, and you have a few more reasons to do multiplayer than in previous games, but that doesn't mean multiplayer is the main way to play it.
We'll have to agree to disagree there. I agree there's a lot of depth to playing 2D beat 'em ups in singleplayer, but I don't think it has to be considered the 'main' way to play if the game was developed and playtested with multiplayer in mind. But then perhaps that's why they're so fun to play singleplayer, because you have to approach challenges differently than you would with an extra person to help with crowd control?
As an aside, it's impossible to convince my friends that STGs are meant to be played in single player. If it has a two player option you have to play that way, they say. Something similar happens here with scrolling beat'em ups, I think.
There are some very good multiplayer shmups, but they're few and far between, and unfortunately a lot don't have team based scoring, so you're basically stuck playing for survival or dealing with a broken scoring system. At least you've found people who play shmups!
On the other hand, if you're playing with someone more serious about shmups some normally single player games offer some interesting incentives to try 'em out in coop, like Giga Wing (where you both need to 1CC to get the good ending, character dialogue/endings for each combination). That's kind of minor compared to Futari, though; it's got some added/altered dialogue when you're playing in 2P mode, but more importantly, you can
abuse the scoring system in really creative ways in Maniac and God due to how the multiplier/cash-in system works.