I'm biased towards all software being open sourced.

Certificate marks look potentially useful if many more people cared about the value of open source (software freedom). People who do care probably already know common licenses, and custom licenses do not inspire any confidence (law ain't easy).

It's difficult to tell when people are internationally misleading others saying "open source" because many devs just say it to mean "you can see the code". Some would sincerely, without ill-intent, call Unreal Engine open source. Would certificate marks promote an understanding?