@MarshallR is quite right, the cost of an XM chip is pohibitive. Sure, somebody is going to buy it, because Intel has made it. However, it is cost-prohibitive for the vast bulk of users, so there isn't much use, especially considering how fast the normal chips are relative to their normal desktop counterparts. Sure, they are slower, but people who need that kind of power (and would spend the money) for an XM chip could buy a desktop that is faster for the price increase of having that XM chip. Just as a comparison, the chip 1 level down from the XM is half the price, and almost as fast:
http://ark.intel.com/products/75131/Intel-Core-i7-4900MQ-Processor-8M-Cache-up-to-3_80-GHz