I think it's worth noting that similar discussions have been held about other systems on other forums. The bottom line is, you can't copyright game mechanics, you can only copyright the text used to explain them. Anyone can create a game using any sort of mechanics they like, and as long as they use their own text to describe the game, it's fine. That goes for computer games too.
You don't see too much of that nowadays because of marketing, which I'll get to in a second, but back in the 80s you saw that all the time with computer games. There were a bunch of computer RPGs in the 80s that basically used the AD&D system, but were not official licensed AD&D products. They simply used their own settings and their own version of the game mechanics and never claimed to be associated with AD&D. The most classic example of this is the old Bard's Tale series.
Having a game be part of the OGL, more than anything, is, or at least was, an instant marketing tool. It was a way to let people know that their game was largely compatible with other OGL games, and it was a way to easily associate their game with D&D and get some marketing from that angle.
If a game or computer game wanted to use a d20-style system without being part of the OGL they certainly could, but then they also couldn't make any references to the OGL or D&D or the d20 system, so they wouldn't get that free marketing boost.
Of course when Wizards created the OGL that was also to help them out too. It encouraged more people to play D&D and it encouraged more publishers to write books for D&D now that they didn't need the express consent of Wizards... they could write their book, slap OGL on it and reference d20 and people would know it was compatible with D&D and use it in their campaigns if they wanted.
