often times the files are incomplete or corrupt, but obviously if someone is looking to "totally delete" data of a disk, it is probably because there is sensitive info in these files, so even recovering part of the file and being able to view part of it is bad news.

Also I won't pretend like I know how the intricacies of the lowest level of the Windows file system work, but I am pretty sure if you do something like open a file, blank out its data, and save it back to disk, it is not a guarantee that the saved data even goes to the same exact physical sectors on the hard drive. So that being said, even blanking out a file may not actually erase its original contents.