An item, for example vfs.fs.size[/net/shared/scripts,pfree] that gets a timeout becomes Not Supported and pre-processing has no chance to run the Custom On Fail action that could save the day.
An example of these errors occur when the file system happens to be on a stale nfs mount. We can create a user parameter and a script for this but I think it is cleaner when the preprocessor gets a chance to check the validity of the data perform it's preprocessor magic.