YES, it is still useful.
Changing default ports has one real purpose only: defend against automated scans/attacks, if you database server is open towards hosts which might get compromised.
While that might not sound like a big deal, remember that:
- any host might get compromised (or your databases server might get exposed to Internet at large due to some mistake)
- most of the attacks those days are automated attacks, and many of them will try only default ports (as aiming at low-hanging fruits is most efficient).
So, yes, while it by itself will not help you much if are under targeted attack, using random ports (and/or making it listen on random IPv6 address only) will make it much less visible, thus at least giving you more time to upgrade before automated 0day exploit scan hits you (and might even protecting you completely against such automated scan all by itself!)
In addition (this will help not only against all automated attacks against, but also against some targeted attacks too) when attackers try to find your database port to exploit it by bruteforce portscans, it can be detected and defended against (by blacklisting attacker IP ranges, and alerting admins if some internal host has been detected as source of the attack)
Also note that changing default port for server and clients (especially if they are deployed automatically) is trivial amount of work, and detecting bruteforce scans is easy too; so you really should be doing it (not just for database servers; but for all services where overhead of setting it up is not prohibitive due to usability issues: like changing default port for web from
80 is not recommended, as some people (and bots) will mess it up, and random firewalls around the world might not allow connection to be established. But RDP is great target for example for non-default port)