another thing these hooks could be used for is backup programs.
today they have to check timestamps, inode numbers, and file checksums to try and figure out what files have changed since the last time they looked.
if the tags are stored persistantly these hooks would give another mechanism
however, alerting when mtime changes is not good enough, compilers are running into problems where they aren't recompiling everything they need to becouse the systems are getting fast enough that the timestamp before and after a change may be the same.
alerting any time that the mtime would be changed, even if it changes to the same value that it was before can work, but you can't count on the mtime changing every time a file is modified.