This. When my friend that have pretty much no experience in fighting games come to my place to play MK, it feels pretty awkward every time. If I go easy on them, they say that they haven't really won, but if I go all in, they get bored quickly because they can't do anything. Having one character against two would be ''kind of'' a good solution to that problem, because right now all I can do is activate the half-HP code and have 50% with each character, but that's much less enjoyable for me than having one single 100% character.
EDIT: Oops, accidentally chose 1v1, which goes in the complete opposite sense than my last paragraph. xD
If I need to choose then 1vs1, I hate being forced to tag.
But I fail to see why there wouldn't be an option. We're not talking about Capcom here with their "artistic integrity" of having shitty stories and 1 game mode.
I find it "cool" to fight someone or be someone who only plays one character. Tag games don't really have mains. And even if the their best character is on their team once they're gone they're gone. I rather fight someone's best character till the end.
2v2 is okay, but, I dunno, I just like 1v1 better. 2v2 can get annoying with switching and it reminds me of endurance rounds and losing because of the second character <.<
I'd say go for a Skullgirls-like tag options, where each player can either choose to play as more powerful single character or multiple weaker character. That way, both parties are catered to.