While I too am a fan of OpenELEC, I need to point out that as you can run XBMC in any distro on any desktop. If media center functionality is the criteria for making a productive desktop suitable for relaxation, then we should keep in mind that with independant media center packages like XBMC, you can do that with any desktop.
Fun OpenELEC story: The first time I installed it, I installed it twice because I was confused by how fast the install went (like 20 seconds, onto a rather old mechanical drive).
KDE can make a lot of things a little more simple for a lot of people who are willing to replace their Windows with something akin the old days of Windows. Having the built-in, 'dont need to install anything yet' functioanlity of a programme that can feed you your media in a simplified manner is (over the usual media player we usually use) is a nice convenience. XBMC is far superior obviously, but most of its stuff isn't required for simple PC/Lappy digestion. It's almost an OS unto its own!
And yeah I was a bit stunned at the install speed on both USB and some crappy old 40GB 2.5" IDE HDD's. Pity ITX mobo's dont ship with 2GB of flash onboard for these kind of things! Thankfully AMD's open source drivers are super mature nowadays and OpenELEC can migrate over and I can stay with AMD offerings.