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.
This has been discussed before (do a search for more info). In general there are (many) progs that do just this, and it isn't really what CC is trying to do. To each his own.
You can also significantly reduce the amount of recoverable data with a few straightforward steps (this is what I do as a matter of routine):
Use CCleaner's secure delete.
Use Eraser to right click erase files instead of right click delete (personally I only ever recommend Eraser 5.7, not 5.8 for reasons far too long to go into here).
Use Recuva once a month to secure delete all recoverable files (which is a hundred times faster than erasing the entire disks free space).