The CheckerBoard

Ok, I'll be the one to ask the dumb question. I watch the game and I see a verion of the 43 most of the time. What are we working on in practice? If we are playing 43 are we practicing a 43 or are we still teaching the 34? If we are spending the practice working on a defense we aren't using, why? If we are a 34 practice, why don't we just run the 34 in the games most of the time because, well, could it get any worse? If I am wrong and we are playing what we are practicing, I apologize for being a bit less informed than most. It really can't be that simple, can it?