Game designers tend to agree that playing games helps you learn about how to design them.
For example, in Rules of Play: Game Design Fundamentals, Katie Salen and Eric Zimmerman write, "Students should play every possible kind of game, digital and non-digital, contemporary and historical, masterpiece and stinker."
They give several good reasons why, including the fact that designers need to learn how games function to create experiences, and they need to see what does and doesn't work about design choices.
Yet, Raph Koster offers a word of warning in his book, A Theory of Fun for Game Design.
He writes, "They [game designers] build up encyclopedic recollections of games past and present, and they then theoretically use these to make new games."
So what's the problem? Essentially, due to the way human brains work, designers are more likely to pull from their existing mental library of game design solutions than they are to try to innovate new ones.
Raph writes, "The most creative and fertile game designers working today tend to be the ones who make a point of not focusing too much on other games for inspiration."
So, the very library of knowledge that designers must build in order to understand and design games can prevent them from exploring new potential game designs.
How do we get around this?
Game designers, of all people, need to "stay ahead of the game." Not playing as many games probably isn't going to help.
Perhaps simply having an awareness of our 'mental game libraries' can help designers choose whether or not to select a solution from them.
Perhaps, too, we can be mindful of fun wherever it occurs. For example, it might be worthwhile to make note when you see yourself or others having fun outside of a formal game environment, and ask yourself how you could bring that experience into a game.