What was going on here when that was written?
Interpolation was added to document the default value only. Originally, VFS_CLASSLOADER_CACHE_DIR just declared System.getProperty("java.io.tmpdir") as part of its value, but that screwed up the auto-generated documentation, by hard-coding the values for the system on which it was built, rather than the values which would actually be used at runtime. This was just a hack to get the documentation correct.
Why do we not want interpolation with user defined settings?
I think we do, but that hasn't been added yet. I think that's best done by transitioning the whole configuration to commons-configuration.