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.