What would be the point of them showing up anyway? We already see all that they want to show us through the livestreams, which are much more interesting and informative than what they would show us at E3. And as for spreading the word to people who haven't heard of PA, they'll probably be too busy admiring CoD's amazing Dog Technology™ to care.
This is how you go to E3, or should I say not, like a boss. http://www.joystiq.com/2013/05/18/ouya-booth-to-be-open-to-the-public-in-a-parking-lot-outside-of/ Not only are you pimping your product, your doing it while giving the finger to your competition and the middlemen.
Good! Better to work on the game I was very annoyed to see Sauropod Studio wasting kickstarter dollars attending PAX to pimp a game that they hadn't even made yet Uber's got their priorities straight