As stated by IANA when one wants to register a port number and a service name in the User Ports range, one of the requirements from RFC 6335 section 8.1.2 is to have a reason:
"...why using a port number in the Dynamic Ports range is unsuitable for the given application."
as I did not find any given examples for valid reasons I tried to come up with one.
The only reason I can think of is the convenience of expecting your port to be free but since no one is really prevented from using your registered port you can't be sure about that. Therefore one needs to have some mechanic in place to resort to a free port from the Dynamic Ports range anyway. Besides that I don't think convenience is a reason they would accept but since there does not seem to be a list of invalid/inadequate reasons I can't be sure about that either.
Does someone know of a list of example reasons or a single example that was accepted in the past or examples that are inadequate?
What are generally adequate reasons why using a port number in the Dynamic Ports range is unsuitable?