It's more relevant than you may think. Lots of embedded devices use XP (Think car diagnostic scanners, Medical diagnostics, etc) - the devices work just fine - and in the case of cars, for example, sometimes there is a special programmer that works for a certain Model/Year of car. Some Oscilloscopes were built on XP... so there's still a use when these devices' hard drives fail or whatnot. Lots of industrial controls use whatever OS of the era they were built.. and they don't go upgrading their embedded systems every time a new Windows comes out. Etc, etc.
It also potentially makes it real easy to use XP in a VM now for software compatibility and whatnot.
On the "security" side, there is always something to be gleaned from previous security implementations, as it usually still has some relation to current security measures; usually an 'evolved' codebase and not a new one... debatable, considering the focus and shift in security since XP up to now.