50$ could make things a little better, but it's far from being just about price. The GC and XBox had a massive price drop, and if it's hard to tell how much effect it had on the long term, the fact is that they could not achieve any real come-back. If it was a long term drop, Sony would ignore if it has not so much impact, and do the same a few month later if it has impact. They are not really in a hurry.
I bet that more than a price drop, what MS want to achieve with that temporary drop is to force more people that wanted to buy it within a year, those who would buy both console later, in order to achieve a good Christmas. I don't think the extra sales have a direct effect on long term (a few millions more or less doesn't change a thing, compared to the 16~18 millions the PS4 will become). But if it sells good enough it could give them the opportunity to communicate about the good Christmas ("Best seller in USA for Christmas", for example), announce sale numbers, and tell basically that the XBone is worth buying. And that something that can have impact on the long term, and improve their image.
