I still have a few boxes with XP on them, mostly for testing purposes. Namely, the software my company develops is aimed at being as resource-friendly as possible. Which implies that it also works on older machines (which should be part of the charm/selling point of our software: just use a machine for it that you thought was useless). Being older machines, it's likely that they also still run XP, so XP compatibility is still an important issue for me.
And well, I personally feel that if it takes virtually no extra effort to support a certain OS/system/platform/whatever, then you should. Even if the platform is not going to be used much, it is quite valuable to test your software on as many configurations as possible, to weed out all weird bugs.
Our software uses Direct3D, and D3D9 on XP is quite a different beast from D3D9(Ex) on Vista+. If you really want to make sure your resource management is working properly (resetting lost devices, re-initializing dynamic resources etc), then you should really test this under XP.