I remember last year's lukewarm reception of the Wii U with the media directly lead to a drop in stock price for Nintendo. Shareholders need a reason to invest money on the company and blind fanboys are not a reason.
Nintendo does have to demonstrate that the Wii U will be well received with both the casuals and the hardcore to give the console legs for at least another 5 years (which seems to be the cycle Nintendo follows). If they don't show a good range of both 1st and 3rd party support we could be seeing a repeat of the events of last year's post E3. I imagine Nintendo would not be looking forward to that.
So long story short, they HAVE to have a good E3 showing, they have no other options because Wii U is their next big thing, it is whats going to make them all that money.
