License costs
Windows based HPC will be a minority sport until Microsoft sort out the licensing. It is from personal experience a total nightmare and not cheap.
A few years back, Microsoft was all gung-ho to try to take on Linux in the high performance computing market, and did a pretty good job with Windows Server 2008 in drawing to performance parity on traditional HPC workloads. But now, in the wake of the massive reorganization by CEO Steve Ballmer, the HPC efforts are being rolled …
Server 2012 outperforms Linux on very high speed and bandwidth IOs (e.g. worlds fastest NFS and CIFS fileservers are on Windows), so for the right work load it is likely worth paying for a license...
Also the expectation would be that the tools are easier and more comprehensive to use if you have to pay for them. Maybe that's not the case now, but I'm sure Microsoft will be moving in that direction...
"it certainly has not become common to have a cluster back-ending an Excel workbook as far as El Reg knows"."
Actually it is very common. For instance in SharePoint sites - or as server powered extensions to worksheets.
http://darrenj.co.uk/2012/01/15/the-power-of-excel-server-side/
http://office.microsoft.com/en-gb/sharepoint-server-help/introduction-to-excel-services-and-excel-web-access-HA010105476.aspx
... everyone knows how to use Windows on a cluster because the the seamless integration with Winphone and Windows RT.
Cluster manufacturers are clearly not showing Microsoft software in its best light because none of them require all data entry to go through a touch screen. All Microsoft have to do is manufacture seven million Windows BC super computers and distribute them exclusively from one Microsoft shop. After spending a billion on advertising, everyone will be queueing up to buy Windows BC because they can be sure that secure boot will protect them from installing unlicensed software. Don't forget - Windows BC is for non-commercial use only. You must buy an upgrade to calculate business spread sheets.