Copyleft Is Not Exclusive
The Copyleft Licence is very important indeed. It regulates the distribution of a copylefted work, to ensure that the work remains open for copying, modification and distribution by all. It's important to get it right. An important point is that any work derived from or incorporating a copylefted work must also be copylefted.
A copyleft licence need not be an exclusive licence. I can still sell copies of my work even though the work is copylefted, under a more traditional scheme. I realise it's unlikely, but someone might want to use one of my poems for a commercial purpose. They can either issue their work under my copyleft licence, which means their work must be copyleft too, so anyone else including competitors can copy, distribute or modify it, or they can issue under a more traditional approach and compensate me. They should balance costs against exclusivity for their requirements; if the traditional copyright benefits are worth the extra money they'd have to pay me, then I'm not going to object.
The same goes for any work copyleft under such a licence. If all the owners of the copyright to the work agree, as is normal under copyright law, then the work can be issued under different terms. Copyleft guarantees the work remains copylefted and available for anyone to copy, modify or distribute, but it is not exclusive. It could get complicated, of course, if a work has been extensively modified, but that's another matter.
Taking this approach works elsewhere. Consider id software and their game engines. At the time of writing, both the Quake 1 and Quake 2 engines are available, as source, from their site, under GNU's copyleft General Public Licence. Any game you produce using the Quake engine with this licence has to be copyleft too. Id software will also allow you to use their engine with your game without distributing your source code, but they'll charge you for it. It's your choice. It makes great commercial sense. You, as a game developer, want to develop your game but don't know if you can get proper funding. Use the Quake engine, and you do know that whether or not you get decent funding, you can get your game out there. Their decision to Copyleft reduces your risks. From id's point of view, more developers will use their engine, so more will pay id to keep their source code secret. If you don't believe me, go to their site, and count the number of games out there using the Quake engines.
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