MS has to release the Xbox one early, it's one of their last chances to 1up the competition. Next thing they need to do is release a special bundle deal with a AAA game or a special colored console.
How much does it really help? If they have tons of stock, maybe. But if they announce the date at PAX, are people going to run out and start preordering? If so, where? Its not like there are a lot of options to buy one at this point if you are really motivated by a 1 week head start. A lot of store stock will be eat up by preorders from weeks/months ago. If stores have stock for shelves, will it be enough to make a huge impact on overall numbers with people who camp out for it?
Its not like Microsoft will be able to create tons of more stock to sell just buy releasing one week early..its only a week, not a month.