Makes perfect sense to me
IBM have done this in the past and it's all part of their "OnDemand" model - I think. We had an iSeries at a company I worked for and were sold on the fact that, if we wanted more power; simply call Big Blue and they'd send a wee update down the line to the machine to crank it up. Fantastic in the retail world where the bulk of your sales, and therefore processing, is at Christmas. The rest of the time, it all depended on whether Tricky Dicky in Merchandising was creating a desktop database or not... You know who you are!
Anyway, the reason you don't have one of these babies on your desktop running Crysis is cost and design. Sure you could run WinXP on a virtual machine on an i-, p- or probably even z-Series, however the machines are designed for out and out processing performance not 3D computation or multimedia. OLAP, OLTP, big database crunches are what they are good at: not replicating real-life environments through physics models.
The reason they aren't used more widely in the science and engineering world is that the software is not available for those sorts of applications. You can write C or even ForTran for an iSeries, so theoretically you can write applications for these sectors; it's just that nobody has really done it yet and to my knowledge.