Almost every good character has a 50/50 minus Cage. Current low tiers like Lao and Tanya and Kenshi don't. Mid tiers like Sub rely on 50/50 corner rape or Erron Black who's got the mix as well. Every current top tier but Cage can mix, but his block pressure makes up for it obviously.
Characters like Alien, Sonya, Mileena, Cuttroat and Demo are characters with a great base moveset AND are given the ability to mix.
Characters like Sub, Scorpion, or Reptile are when characters have a strong 50/50 OR (as in not both) strong base set.
Characters like Kenjutsu or Cryomancer where you can mix but your neutral is dog shit.
So to say it is only mixing that is good isn't really right imo, but it should also be said that most of the better characters in the game have them it can't be a coincidence.