If somebody learns that such-and-such DynDNS URL is mine, they can always do a DNS lookup to find my current IP address. They can:

- find my approximate physical location

- regularly scan ports on my IP and wait for me to accidentally expose a service I didn't mean to

- track me in any access logs they have access to

This requires no port forwarding, so someone brute force scanning your IP won't find this and your open port. they also might have a harder time figuring out what service has been exposed since the port number itself will be unknown.