Named instance configured on dynamic default port
When would it ever be useful to operate named instances on default ports?

I cannot think of one and Microsoft does not recommend doing so.  When you are connecting to a named instance through a firewall, it is recommended that Database Engine to listen on a specific port, so that the appropriate port can be opened in the firewall. That should be something other than 1433.  With dynamic port does not guarantee that SQL Server will always listen to the port you specify.  If some other process already using that port dynamic port will be set to zero and another port will be assigned.


  We do not recommend that you have a named instance listen on port
  1433.


What happens when a named instance listening on port 1433?


  If a Named Instance of SQL Server is listening on port 1433, the
  Microsoft OLE DB Provider for SQL Server (Provider=SQLOLEDB) and the
  SQL Server ODBC Driver (Driver={SQL Server}) may be unable to connect
  to the server. This failure to connect occurs when both the instance
  name and the port number from the connection string are omitted. If no
  port number is specified, the drivers validate the server instance
  name. If the server instance name is not the default instance,
  MSSQLSERVER, the connection is unsuccessful. To enable a successful
  connection, you must specify the instance name or the port number in
  the connection string or in a SQL Alias.


If somebody want to use a specific port number for SQL Server named instance they should consult this and pick a number not listed here. 1433 is listed for SQL Server default instance.

why would it ever make sense for them to be dynamic?

I am just making a wild guess. If they install a default instance 1433 will be used by that so dynamic port setting will pick some other port.

Ref:


https://support.microsoft.com/en-us/help/823938/how-to-configure-sql-server-to-listen-on-a-specific-port
https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml