You spot on with gaming companies (especially Sony and MS) backing themselves into a corner by constantly trying to have something big and new to show every year.
But there's nothing wrong with that approach provided you actual have the games to show something yearly. I just think the focus needs to be on gameplay demos and not CGI trailers or essentially trailers of trailers (wtf) whatever it is that they showing.
Now that Sony no longer have major press conferences they have essentially moved to the Nintendo model - show something when you have something and make it focused! I commend MS for keeping E3 alive though and I feel that the right and most effective approach is probably a combo of the two.