I'm no expert in P4A, let alone MvC2 or MvC3 (I'm alright in MvC:Clash of Super Heroes). But I would imagine higher level combos in P4A would require your active character's persona on screen to help extend combos and what not. Calling in a character assist to extended a combo MvC style would still make it so that there is potential for 6 characters on screen.
This might be where you're confused, because that's not quite how it works for extended combos (minus S. Lab, who is a puppet character so inherently requires the constant presence of Asterius).
Personas in P4A do not stay on screen for extended periods of time except for maybe with a few characters like Liz or Yukiko, but extended combos will have a character pull off normals, their Persona will appear to do something then disappear, pull off more normals and their Persona will appear again... That's mainly the rhythm.
If you truly make a Persona MvC where you can only call in ONE Persona OR character assist at a time, I think it would lose some appeal. Part of what makes P4A so cool is that you are doing combos between Persona User and Persona. I don't think it would be as awesome if it was just Kanji assisting Yu pick up combos.
And that's partly why I'm not into the idea of teams applied to the P4A formula.