In looking over some of the game designer job descriptions and interviews that are online, it seems to me that a game designer has to do/know a little bit of everything. They write books/novels/comics, design, consult, collaborate with others (on a limited basis), are familiar with a variety of technologies (3D, C, C+), and play games. They are also politically and legally savvy.
For example,
Michael A. Stackpole [BattleTech, and Star Wars X-Wing comics] posts some of the questions that he is asked:

Question: Do you play BattleTech?
Answer: "I have played three times, I've won each time, so, I've retired undefeated."
He explains that when the books came out-he was considered to be an expert and players would seek him out to kill him or run away. No fun!! He goes on to say that he rolls the dice to see if shots hit and to determine where they hit, so he knows his way around the combat resolution tables for the game.

He says, "Before I started writing the BattleTech novels I read the rules, moved the 'mechs about on a board game and conducted combats by myself. That gave me the basics of how the game worked and folks at FASA gave me some insights on finer points of strategy."

On the legal side, he does not read other works because he does not want to be accused of plagerizing someone else's work--in case he has a similiar idea.