One question: Is Obama even going to be there? After all it's going to be on inauguration night. Lots of stuff going on that night...
Seems to me that giving a military-only ball and yet being too busy to show up would kill two birds with one stone:One one hand you calm leftist 'fears', and on the other you throw a sop to the military and get great PR. I mean, who can blame you if you couldn't show up (or stay more than a few minutes)? It's the night of you inauguration, for Pete's sake.
They'll only stay a few minutes at each of the balls they go to. Each state has a ball, assorted organizations do too. Every ballroom and large-enough space in Washington will be in use that evening. The President and First Lady can only go to a few of the many, and they will not stay long at any of them. Since Obama is specifically continuing this 'tradition' (it's only the second one), I expect he will put in an appearance.