Although it seems like a minor thing for people outside the beltway, the dinner is a social highlight for many of the Beltway's most prominent journalists. The President's decision to ignore it carries with it a faint whiff of ingratitude, given that he's received over-the-top adoring press coverage -- but then again, it's just part of a time-honored pattern of Obama stiff-arming his sycophants in the media, as he did during the campaign.
As we all know, his treatment of the press didn't hurt campaign coverage of him at all. But now, it may be different, and the decision may carry some risk.
Memes started by the national media inside-the-beltway bit by bit leak out into the country at large. That's why it's dangerous for Obama to have "the establishment" doubting his ability to do his job.
Occasions like the Gridiron Dinner represent opportunities to shore up relationships with people who can (at least on the margins) help or hurt a president. The fact that Obama isn't taking advantage of such an opportunity demonstrates one of two things: Either he doesn't realize the importance of such seemingly small gestures, or else, his self-confidence is such that he doesn't believe that he needs the help of the press to succeed.