Ok, so let's go then. Not having a full press conference at E3 is attracting some bad media coverage now. They already knew that before announcint that it would be this way. Looking at the coverage, they know MS and Sony announcements will gather public attention and whatever they announce won't get as much attention because of the lack of the live conference. So they are losing a good marketing opportunity here to appeal to the gamer public and a great chance to appeal to the casual public, since E3 as the biggest yearly conference for electronics will get some time in tradicional media like TV and will reach a portion of the larger public. This marketing is what they get (or in this case lose) with a live conference.

The downsides for a live conference are very few. Cost? Pretty much nothing for any company. Live mistakes? This kind of thing isn't that common, despite minor mistakes here and there that won't get much more thansome laughs. Bad reactions to live conferences aren't because the presentation was bad, but because what was presented was (like the reaction to MS last E3) and doing it live or not won't change the reaction in this case. So, nothing to lose and a marketing opportunity to gain.

Another factor we can consider is that all big announcements are made on big events. That's how Apple shows the iPhone, Samsung shows the S phones and it's how PS4 and One were announced. You can even tease this things or even say that they are coming (basically stating the obvious) just to get coverage and attention to the big and real announcement. And that has to be made live.

And I'm not stating Nintendo is inferior. I even said they have a great and anticipated game to show (SSB). But the public perception when you aren't there is that you don't have nothing to show. Each company has to make the consumers know their product and what they have to offer and Nintendo simply isn't willing to do it at the biggest electronic expo of the year by the second straight time. 

no to all. they will get the same atention or more than a live conference.

Part of that is message control, something Nintendo is notoriously poor at. They do their thing, people make conclusions about that, often quite negative, and Nintendo just lets these impressions run amok in the wild. If Nintendo stayed ahead of the message, it wouldn't be damaging.