Interesting, but there’s a fundamental causation vs. correlation problem here. I know at least one person who says she doesn’t play RPGs because she’s not imaginative enough.
A study forthcoming in Thinking Skills and Creativity found that people who play table-top role playing games (e.g. Dungeons and Dragons) engage in more divergent thinking (a common measure of creativity) than people who play electronic role playing games (e.g. Final Fantasy) or people who don’t play any role playing games.
What makes a game like Dungeons and Dragons so beneficial is that it gets at the cognitive core of what creativity is about — the act of connecting existing knowledge in a novel way in order to generate new knowledge. This new knowledge can be a pleasant way to place paint on a canvass, a plan to stop the leak in your sink, or a way to explain how a Dwarf’s Level 3 Fire spell is repelled by a Dark Ogre.