No need to care about marketshare at this point. Not at all because all the potential hardcore gamers already chose their platform a long time ago and therefore there's not many costumers left who you'd want to tie to your echo-system.
I think Sony's price-cut is stupid because they're losing free money just to get some meaningless marketshare at the end of a generation.
Let's say MS makes $100 in profit off of each X360 sold (yeah, it's most likely not that much but a smaller number only supports my argument even more). If they did the usual $50 price cut that profit would shrink to $50 on each unit.
Let's say without a price-cut MS sells 9 million X360's and with a price-cut at $50 they would sell 11 million in the following year.
Now do the math.
9 million x $100 = $900 million
11 million x $50 = $550 million
Even with the profits from more SW sales and Xbox Live subs those price sensitive 2 million extra customers cant bring in $350 mill in profits this late in a generation.