Well, if they all just use their own movesets (which look to be good, at the very least) then the boss battles themselves will probably end up being just as serviceable.Part of me hopes all the playable characters are bosses, but I also wonder if it's possible to make good boss battles out of each of them. I'd rather they just have one good one as opposed to four which are mediocre.
But yeah, I agree. Better to have one good Vergil boss battle than four so-so ones.