I tend to disagree. Yes slammer hit 1433, but in my humble opinion you do not change port numbers due to security issues. If you do read about slammer, the patch for the slammer security issue was released more than six months earlier. Slammer did much damage to to unpatched systems, not due to default port numbers.
There are still valid reasons for specifying port numbers. The two most common cases I've seen is routing access through a firewall (were all ports are closed by default), and access to named instances from clients who don't understand instances (by using IP and port number).
It's true that there was already a patch for Slammer, but some people prefer to add that extra layer of security as well. What would have happened if another worm that there was no patch for had come along exploiting 1433? I know that not everybody believes in security by obscurity (changing names and numbers away from default or descriptive values in order to mask their purpose), but it's a genuine school of thought.
I happen to think that changing ports for security reasons is a good idea, but of course I understand that not everybody has the same point of view. I only pointed out that this is a reason why one might wish to change the port.
Thanks for also pointing out those other reasons.