That's where the designers skill come in play.
If the game design is bad then it will need to be changed during development.. Or created if it's not very detailed.
I find it so extremely unprofessional for teams to completely change their game design in middle of development.
To be able to take into account all possible programming features and limitations in advance, you need to be a really good programmer.
To be able to decide exactly how your artist would draw the art for the game, you need to be an artist. How else would you write detailed instructions on drawing, for example, a knight who took an arrow to the knee?
So, are you saying that Programming and Art are two vital skills that a game designer would need to have?