Sure, it'll do nothing to stop a determined attacker, but it does wonders to stop the noise from passive scanners.

Are you familiar with the Swiss cheese model of risk management[0]? Obscurity is just another slice of Swiss cheese. It's not your only security measure. You still use all the other measures.

[0] https://en.wikipedia.org/wiki/Swiss_cheese_model

It will conserve a lot of defender resources, it will completely bypass all mass scans, and it will make "determined attackers" much more visible as they will have to find the port first which will show up in logs and potentially land them in a tarpit.