Paul Gienger wrote:

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Modifying a file and deleting it are two entirely different things. If you create or delete a file, you're modifying the file that describes the directory contents, not the contents of the directory. The most you can do, if you can't delete the file, is reduce it to zero bytes. However, you won't be able to delete the file.

Since I think we've crossed into the theoretical here and completely hijacked the OP, what use could this extra bit possibly have? If the data is gone then the file just as well might be gone. I would have to think this is probably close to the reason that there's only rwx in UNIX, regardless of how many complex ACLs you have.

I'm not discounting the possibility that the rwx+m permission structure has a valid use-case, I'm just really struggling to find it. Can anyone elaborate a good use?

It also prevents someone from creating a file. The whole idea is that one person can control what files are added or deleted in a directory, while allowing others to make changes to those files.