That’s probably a majority of the point. Falsely report that some interesting ports are open and he’ll spend time on them and potentially trigger alerts or blocks.
Fake open ports aren’t something a normal user would bother with or understand, but with all the tools available in the nefarious side, it makes sense to have options that make their job harder if you’re willing to use them.
Maybe what you’re referring to is along the lines of a port being open but the software on the other side of it not sending acknowledging responses?
At a guess, you might tell the difference between some benign scan and an attempt to actually take advantage of the port, perhaps to use as a trigger to automatically ban an ip address? or a way to divert malicious resources to an easy looking target so they are less available in other areas?
The difference between someone scanning for open ports and someone attacking a port they find open seems significant enough to at least track and watch for patterns… Whether that’s useful for the majority of users or not is rarely why a feature is implemented.
A benign scan could just be looking for an ftp server to connect to or a repeater or relay server of some sort. There are plenty of open services people make available for free and the fact that you would consider it an attack it doesn’t make it one.
At minimum you could be alerted to look for someone attempting to connect to your ftp server with a single basic anonymous authentication vs someone flooding that port with known malicious software attacks, and block the latter across your entire network and effectively ignore the former. Really it seems like you’re advertising your lack of imagination in this context than a legitimate lack of possible uses for spoofing open ports.