Consider a browser cache. /tmp is not guaranteed to be preserved between program invocations[1]. /var/tmp [2] might be a better place. /var/cache [3] is intended for exactly this kind of thing.
Unfortunately, it can be far more useful for an administrator in a multi-user setup to want user-specific caches in home directories. That way, you get all of the infrastructure for managing user data for free, like quotas.
Also note: if you're considering putting data into /tmp or /var/tmp, please honor the TMPDIR environment variable, if it is set[4].
[1]: http://www.pathname.com/fhs/pub/fhs-2.3.html#TMPTEMPORARYFIL...
[2]: http://www.pathname.com/fhs/pub/fhs-2.3.html#VARTMPTEMPORARY...
[3]: http://www.pathname.com/fhs/pub/fhs-2.3.html#VARCACHEAPPLICA...
[4]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_...
What fraction of those requests are by XDG advocates?
I ask because every standard comes with folks insisting that it be followed. While those folks claim to represent the interests of users, those requests are different.
A standard that uses environment variables means programs don't have to provide extra options for this customization (I've seen -f,--file , -c,--config and other variants). It allows for common code (libraries that implement the spec).
If you poke around for feature requests for various open source programs, you'll find XDG basedir compliance come up occasionally (moreso for CLI utils). I wouldn't say "nobody seems to be using it"; quick scan of my folders includes Chromium, uzbl, htop. Git's next release will be compliant too.