1) Microsoft can't beat Linux's price for HPC. Even $50 a piece adds up when you have 1,000 boxes. $50,000 more for what benefit?
2) Microsoft can't make it as easy as Apple has to set up and manage a cluster. Right now you can get HPC cluster systems out-of-the-box from Apple for relatively cheap (ready-to-go 16-node 32-proc starts at $60K with support), and be running HPC applications with little knowledge of clustering.
3) The only place where this might be wanted is those few places that for some reason put huge amounts of data and calculations on Excel spreadsheets. Any Excel spreadsheet that can't be quickly calculated by a modern fast Opteron system needs its data to be put in a different format.
Huge Excel spreadsheets? Uh, isn't that what databases are for?
Plus you have to factor in the overall cost if you have, for example, five computers. Normally a Linux distro is licensed to the END USER, not the computer, while a Windows license is for ONE COMPUTER. I run Xandros 3.0 Deluxe on my laptop and desktop...the Xandros license allows me to install my copy of Xandros 3.0 Deluxe on as many computers that I want to. If I wanted to install Windows XP on five computers, technically and legally I would have to buy five separate Windows XP licenses. Having Xandros 3.0 Deluxe on five computers would cost you just $90 (Xandros 3.0 Deluxe retails for $89.99), while having Windows XP on the same five computers would cost about $1,000. And then if you wanted an office suite on five computers, OpenOffice.org would be free, Sun StarOffice costs about $70 but the license is to the user, not the computer, while Microsoft Office on the same five computers would run about $2,500 (Office retails brand new for either $400 or $500).