If this is not done by CCleaner I think it would be a great safety feature.
As far as I know free or available remaining space on a HDD is not truly blank but contains recoverable information that may be of a sensitive or private nature.
A facility to securely clean, over write or "format" the unused portion of a HDD only, making the free portion of a HDD truly clean would be a valuable tool for those who store or have stored privileged information on a PC, notebook or other. Without such a tool his information could be recovered and used in the event that a pc is lost, stolen or violated.
I look forward to comments in this regard.