While it is not a reserved port, 5665 is probably well-known. So if the port is open, a scan will quite easily reveal an installation of Icinga 2. So if there is some vulnerability and it is exposed to the public this is more critical than normal. Also I am pretty sure that scans already result in a bloated log, someone can easily request a certificate and if erroneously signed get too much information about the environment without an easy revocation of the certificate, and many more scenarios which makes me think it is not a good idea.