At Readercon this weekend, China Mieville said, in his guest of honor interview, that one of the things he notices in both the audience for his work and in himself is a tension between a desire for otherworldly mystery and a desire for detail, detail, detail. He noted RPGs as an expression of this tension, a sublimation of geekiness within the rules and tables and worldbooks of the game that was often at odds with the fantastic potential of the material, and sometimes of the source material itself — he noted that the game of Call of Cthulhu seemed to utterly miss Lovecraft’s point: Cthulhu goes from being a creature so great and terrible that it can’t possibly be described or comprehended to being a creature with 100 hit points. (I may be mangling China’s argument, since it’s based on memory, so please blame me if you disagree, not him.)

The comments are worth a read, too, including this one from Cheryl (I’m assuming Cheryl Morgan):

I could go on at far greater length than you would want about role-playing games, but basically it all boils down to a philsophical difference between those who play them regarding the purpose of the mechanisms. Some people see them as a set of game rules that are there to be learned and exploited, just like the rules of any board game. Others see them simply as a useful underpinning to the simulation that should not be allowed to get in the way of the experience.

The argument you put forward is valid, but you might also ask whether an artist’s impression of Cthulhu (or a plush Cthulhu toy) devalues the true, mindboggling awfulness of Lovecraft’s creation. The answer is, “only if you let it.”