Can you explain what you mean by this?
I haven't played DBF, but I thought they functioned the same as MvC2/3. Did you mean Infinite?
Or do you mean the fact MvC characters had 3 assists that you could chose from before the fight allowed for some level of variation (even though most characters had 1 assist that was absurdly better than the other 2)?
Like Sandbox said, it's better for a character to have one assist that was overwhelmingly better than the other two options over just having one assist that's not even good at all. And even in Marvel you have a few characters that had more than one good assist like Dr.Doom.