In case Joel's 'not really' is confusing, his point is that you save the same amount of space by erasing it as by deleting it.
I detect some misunderstanding about what Eraser does. It does not make space any more 'empty' that it was previously. What it does, as Joel says, is overwrite the data that would be left in the space marked as free if a file were only deleted. The space is still free, but there is no more of it than if the file had been deleted.
This is a simple explanation. There are complications relating to Restore Points and the like, but I won't go into those.