Stumbled across this thread via search and just wanted to add my 2 cents with regards to this feature:
In my case, the automatic port selection conflicted with a Docker container that was already using port 5900. When I start the VM manually, it works fine because it selects 5901 and 5902 for VNC and Spice. However, when using autostart, it selects 5900 for VNC, which makes the Docker container fail.
Either way, I need to reconfigure either the Docker container or change the ports on the VM. Of course, the latter is possible through XML editing, but I was still surprised to see no easier method in the plugin's UI. The ports are displayed in the VM list, so I did sort of expect to see a "change port" or "edit" button or something. Or maybe to set an "allowed" range of ports, so I can better avoid these conflicts in the future.
Anyway, it's not a big deal, just a usability nitpick.