to hurt the dominant opposition.  Sony has a loyalty advantage.  Fanboys are stupid enough to go out and buy it at any cost, just to say they have a PS3.
While you are probably right, the gamble to hurt Sony might pay off in software sales.  Lets say that they lower the price by $50 and thus a new buyer says, I guess I'll get another game for $60.  Assuming MS makes $10 of the game, they only lost $40 on that console.  They look $50 cheaper, only take a loss of $40 and stop one more sale of a PS3.  I can see them doing it, because MS is nasty like that.
you still have to think about the ramifications of that $40 loss... sell another 5 million units with that price structure and MS just lost another $200M on TOP of whatever they are already losing on each system.  That one game sale means nothign to them.
And in regards to stopping the sale of a PS3, as was mentioned, they already have a nice foothold in that argument.  They are $200 cheaper than the PS3 out of the gate.  Kids will have a much harder time saving up for a PS3, so they may get a 360 instead.  Parents will get their kids a new system... but why not save $200 by getting a 360?  People like us, even if we really WANT a PS3, are likely to hold off because of the high price.    I'd say MS is in a nice situation in regards to that.  A price drop would make the 360 look slightly better... but IMO, there is no business sense behind the move.
I still say they will go with a bundle.  Games cost next to nothing compared to systems.  Even peripherals are cheap comparatively.  They can throw in 2 or 3 games even, make it a really nice bundle... and they'll look a ton better than a bare $600 PS3 with no games.  Plus... with a price drop, you are stuck into it for the life of they system.  Short of a rebate or similar offer, you'll never be able to go back to a more expensive price after a price drop.   With a bundle, you can offer it for a few months to steal Sony's thunder, and then take it off if it is hurting your bottom line.
Microsoft is a lot of things, but they aren't stupid.