question of the day: Does it make any difference to ‘Black Swan’ if Natalie Portman did not do most of the dancing herself?

I never thought Mark Hamill was really flying an X-wing. I never thought Ray Liotta was actually a gangster. But the collective power of the movie surrounding them created the illusion that they were. Some of that illusion is created by actors, some by special FX, some by other people talented in myriad ways. Surely what matters is the total illusion, not the breakdown of who created which part of it. Or am I wrong?