Ergonomics. I don't like having the mouse way off to the side (or in front of the keyboard), even though I rarely use it except when browsing the web. I never enter numbers in large quantities. Btw, I also work as a developer/sysadmin.
So far, I'm very satisfied with my Filco Tenkeyless Brown...
More or less. If your computer's BIOS supports booting from the network it will probably use PXE to load the OS from a TFTP server. By loading gPXE instead, commonly referred to as chainloading, you get much better support for e.g. iSCSI. It's also possible to use Proxy-DHCP to supply the PXE...
I just got a reply from The Keyboard Company saying that they will have Filco Tenkeyless with EU layouts (UK, DE, FR, SE/FI, ...) some time later this year or early 2011. I can barely wait!