What? Don't be silly.
BC would be a boon for MS, I'm sure it would be a relatively big selling point against Sony. the problem is execution and whether or not it would be hardware or software based. Software based is pretty much pointless, sure you can get games running but they often see bugs, glitches, etc that make playing those games annoying and ultimately pointless.
If they can manage hardware BC, I'll be very surprised and pleased.