[rbanffy]

If you don't want to see a proprietary fork of your code, you should use GPL. If you don't mind seeing proprietary forks of your code, you can use BSD, Apache, MIT or any other more "permissive" license.

I don't think Plan9 has much commercial value in it left, but they may consider it would be awkward if, say, EMC or Cisco incorporated some of its technologies (Plan 9 had some interesting ideas about storage and network transparency) into their proprietary products without paying for them. It's easy to sympathize with the small garage inventor and to think it's nice for you to subsidize small innovators, but it's much harder to do the same with corporations that would patent-extort you the moment they realized they could gain some advantage from it.