Interesting. It is true that after you have wiped the free space, once you use your pc again, the free space will become occupied to some extent, but this can be minimized by regular wiping.
Using Webroot Window Washer to wipe the free space, the first time you do it all 100 % of the free space is wiped. But next time you wipe the free space, it only wipes about 20 % as the rest is still unoccupied after the initial wiping. The original wipe can take about 10 minutes on a small 20 gb hard drive, but the subsequent wipes of the 20 % or so re-occupied space will only take about 2 minutes.
Keeping the free space as empty as possible should speed up the pc and also limit any security risk from info contained in the free space.
What would be ideal is a wiping function with a scheduler. Eraser does have this, but compared to Window Washer (which does not have a scheduler for wiping the free space) it is quite slow.
The actual CCleaner cleaner and Issues fixer are really fast compared to most other programs, so I expect a Wipe Free Space function would be equally fast.