Here's one good reason: creating disk images.
I dual boot Windows and Linux and one of the things I really appreciate being able to do is create a block-level backup images of a fresh, properly updated and validated Windows partition as protection against corruption. (I just had to rebuild a friend's machine after a few bad sectors happened to take out the system hive of the registry, among other things.)
When creating a disk image, ZEROs compress much, much better than random noise or old data. I want a tool that will erase data not just for security, but so that any part of the drive that isn't actual data compresses to almost nothing very easily. Right now, I'm stuck with a disk image that is almost 10GB in size COMPRESSED when there are less than 5GB of data used on the disk. The MFT could be a sizable portion of that overhead, especially if some well-intended eraser program fills it with random noise instead of nice, clean zeros.
I can blank the unused blocks with Linux commands, but I'm not sure I can black out unused portions of the MFT the same way. That is at least one reason to have the "MFT only" option, but I wouldn't mind at all if it was included in a full wipe.
