This is the 'perfect world' scenario where everyone does 'the right thing', of course in such a world we wouldn't need any laws or any written contracts at all.

In reality though, I make sure that I have a written agreement with my employer which legally binds him to pay me a specified salary at the end of each month, because when push comes to shove I don't blindingly trust that he pay me my salary just because it's the 'right thing'.

Nor do I only want my specified salary if he 'wants to give it to me instead of being legally bound to do so', if he owes me salary I want it even if he doesn't want to give it to me.

In my opinion when it comes to companies in particular (which is generally the equivalent of an extremely selfish person), the typical course of action is to only contribute back if they legally have to, or if they see a practical benefit in doing so that outweighs the potential advantage they could have by keeping their changes to themselves, color me cynical.

I also think this is what makes copyleft a good base for cooperative development between said companies, as they are all legally bound to contribute their changes back which is something I believe comes across better in the board room as opposed to relying on other companies returning the favour because it's the 'right thing' to do.

>Struggling to come up with a single category of software where this would merit derivatives include source sharing provisions in the license.

Any project where you want to be able to benefit from any enhancements made to the code of the original project, including any derivatives/forks.