2008 highlighted how insignificant E3 actually is because it was Nintendo's best year by miles as sales go and sales even increased after E3 2008, it showed that E3 had become nothing but daydream for forum goers.
It may not have affected sales. But I would argue that had more to do with E3's short-lived "Business and Media Summit" format, which greatly reduced the hype and awareness of the actual event. Low attendances meant less companies brought stuff worth getting hyped about. You're correct that they don't impact sales, but they do help bring awareness. Thankfully the ESA realized this was a stupid idea, and reverted E3 back to the way it was in 2009.