It does have advantages for sure.
* can dnf downgrade easily.
* can choose not to upgrade something thats a big change you aren't
I don't know enough about RPM packaging or DNF CoW to say either way, but I wonder if these enhancements might also contribute to getting DNF CoW's "reusable extents" feature working? If I understood the FOSDEM presentation correctly, DNF CoW would not only save the installer from having to rewrite the file to the user's disk on update when the file has not changed, it would also not download the file in the first place if it hasn't changed. Presumably this depends on the same functionality that deltarpm would need to identify unchanged files in RPM packages? So if these enhancements to deltarpm would also contribute to DNF CoW, I would consider that another plus.
Here is the DNF CoW FOSDEM presentation if anyone is interested in it.
Just my two cents.