Look, just about every single console launch in recent history was accompanied by shortages. LOAF already explained the reasons in this thread - you can't just spike up production for a few months, because to do that, you have to have the infrastructure for it... and if you have the infrastructure for it, you'll be using it, because it'd be a waste of money to just let a production line work at 50% capacity or whatever. In other words, once you increase production, it has to be a permanent increase, otherwise you lose money.
Considering that a shortage would have occured no matter what, ask yourself this - would Microsoft have gotten less criticism had it lied, claiming that there's more than enough Xboxes to go around, when this clearly wasn't true? Under these circumstances, to blather on about how Microsoft benefits from these rumours is downright stupid - the rumours were a consequence of the situation. Microsoft didn't do anything to start them, it merely tried to control them by being forthright and honest about it.
Also, this publicity is anything but free - as many press articles point out, Microsoft is facing a very real risk that customers, irritated by the shortages, will give up on buying the Xbox altogether. Certainly, the parents who want to buy their kids an Xbox for Christmas are a lot less likely to buy one after Christmas if forced to do so due to shortages.