If your users feel better about your app, they are more likely to tell others about your app. That's far more likely to get you found and purchased than popping slightly more than the next guy in the app store. That said, it's not clear just how big an impact a cleaner icon has, and the likelihood that your users will be talking about you in the first place probably depends on what your app does, &c. It does seem a good thing to keep in mind, though.