Besides promotion to customers, E3 is typically a networking type of event for a lot of people in the industry. If there isn't a booth, would Sony bother sending anybody to schmooz and shoot the shit with people in the industry?
Like, if Sony copies Nintendo, there's a booth, games, Sony people, but no conference, that's not really a biggy. If no effort is put into E3 at all, it's a missed opportunity that isn't everything but its still important.