Personally, I don't think you have to make a big case for allocating
another port. Indeed, I would be surprised if you wanted to reuse
the IPP port for notifications.
A server that receives notifications is different than a printer server.
If you were to operate both on the same host, you might want them to
be differnet processes (perhaps even supplied by different vendors),
which implies that they need to be different ports.
This is just off the top of my head. Of course, if you've got good
reasons for reusing the IPP port, those would need to be considered also.