Strange and wizardly. LOL.
Yea this alone is a great reason. He was all over reddit during the OGL saying no one would ever need to worry about that, but:
1- He could change his mind technically.
2- He could die or be found liable in some court thing and then his stuff gets sold to the highest bidder, who then does exactly that
3- Some corrupt company could get through some ruling where suddenly it's ok to copyright game mechanics actually, and now the fact that you based something on stars without number means that whomever manages to get the rights has an extremely good case. So basically (1) or (2) plus a further corruption of the system.
All of these are headed off (as much as possible) by a license. He's not at all wrong that such a license isn't strictly necessary- much as the EFF was correct when they pointed out that all the crap Hasbro was pretending to license with the OGL isn't actually copyrightable to begin with. That's nice and fine, but how much does it cost to prove that in court? And do you want to spend a serious fraction of your life winning that battle- or losing it, should it be discovered that you were once opposed to transwomen competing in woman's bodybuilding, or whatever?