Well this is mostly from my experience in the past, but now I think I got enough confirmation that I can formulate this as a theory, at least regarding game development:
Proprietary developers benefit the open source development more, than the idealistic people dedicated to open source development.
In the programming world, there are different camps with different ideologies, like conservative people who do it for money and make proprietary software and different open source communities, from BSD philosophy to GPL philosophy. So a little backstory, when I began game development ( with Torque 3D ) I thought now that this great engine is open source, probably a lot of people from the various open source communities will use it, since it was the best open source game engine ever to be released in computing history, but I was wrong, almost nobody came. Not only did nobody came, even when I tried to encourage people or talk to them for help regarding programming problems, they never were helpful. Quite the opposite was true with the regular community that was with the engine before, they were engaged and motivated and also willing to help, even though they were all doing closed source proprietary projects. Not only were those proprietary developers more friendly and more helpful to newbies, they also were more liberal about their help regarding licensing than almost any open source idealist, since they did not even bother to add a license to their work or demand attribution.
It seems to be quite common even in the proprietary commercial work to just reveal your secrets, even big companies often release paper on their technology and how they did it, of course they are not releasing everything, but the basic mechanic. The algorithms are basically public domain, I mean you would need patents to protect those things anyway, regular copyright cannot protect a way to do things. One proprietary developer once said, that it is not a big deal to tell how it is made, the work is in the implementation and if you can write an implementation and optimize it, you can have all the credit.
In communities around a software like a game engine in my case, it also seems to be normal to contribute back to the community, which then is basically public domain content, or in my case it will become the same license as the original software, so everyone who owns the software also owns all the add ons and contributions towards it from the community, which in case the original software is open source as well is pretty cool, since you never need to buy anything. However also with proprietary products this mentality is common, since user contributions will enhance the value of the product for the company, so the company is interested in as many contributions as possible and the contributions have to be as liberally licensed as possible, so people can use them, which results in everything being basically public domain, if you own the original product.
If I watch the open source communities now, their policies seem to be quite different and their end results as well. The first thing most open source idealists ( especially GPL people ) think about is the license and who gets the credit and who can use it how, why and what you cannot do with it etc. The difference is basically the open source communities bother with licenses first, the other proprietary ones bother with licenses last. This results of course in many cases, that nothing gets ever done and even if, the quality and quantity is bad and on top of that, the license added to it is worse than what you get with proprietary software ( if you ignore the little restrictions that the base product is proprietary ). Yes many open source idealists may complain now like "But the proprietary stuff is proprietary and not really free" - Yes that is correct ultimatively, there is a copyright barrier that restricts you, since of course the company tries to bind their customers to their products and services, so there has to be some kind of restriction. That is the tactic of user lock in, they try to keep their users in a sandbox, but within that sandbox everything is much better and more liberal as people within the sandbox share much more.
Well in my case such a proprietary sandbox as a whole got open sourced, so the initial copyright barrier that locked users in got removed and what was left, was all the benefits without all the drawbacks. Well back to my theory, I think that still, even though there are always some barriers to lock users in, aka make them unfree, that what is left is still more beneficial to open source as a whole, since even commercial companies usually producing only proprietary stuff, sometimes give something back and if they do, it is much better than the open source idealists, who often already fail at the production stage, so they are often not even left with anything that they could contribute. Another case is that companys fully fund open source development, since they need it for their business, or a company goes bankrupt and then source dumps everything, so it at least survives as a product and gets maintained.
However my initial point was, that I experienced people comming from the proprietary world producing proprietary commercial products, seem to be more friendly, helpful and especially more productive towards open source software development, than the open source idealists ( probably due to the open source idealists being nonexistent for the most part ). This is kind of an add on regarding my article about why I'm not for GPL license, showing how unproductive it is in reality. The other open source factions are more productive, but still less productive than the proprietary world. Of course there are big exceptions where open source development is very productive and high quality, but this is mostly due to big commercial companies donating all the money for it, since long term people usually do not work for free.
The other exception are of course art assets, since art assets are usually proprietary and no proprietary developer ever shares them as open source, they are liberal with sharing relatively often, but then only under certain license conditions that are not open source compatible. The art is what makes their products image, at least in case of games so they will not share it. Sometimes they will share base material, but hardly ever finished assets. So in this case the open source community is clearly better regarding sharing free art, but I was focusing here on the programming side, since with the exception of games programs do not need much art assets. But regarding the usefulness of open source game art, I also wrote an article, which conlcuded that it may not be that useful at all.
It seems the proprietary developers are always the leading factor and the most productive, even for open source software, since even their leftovers are better than what most open source idealists produce. The most obvious factor why this may be is probably money, since money makes the world go round (and if you GPL your project, you get no more money). But there are also other factors and that is hierarchy and leadership, commercial companies are total dictatorships, the leader tells the direction and then everyone focuses on achieving that goal, while the open source community is mostly disorganized and anarchy, so everyone splits off their own projects when someone disagrees with them and you are left with lots of small branches of a project that never get big in quantity or quality, since there is just not enough manpower and direction.
It may sound a bit depressing to realize that the open source idealisms do not work for the most part and the open source community has to be satisfied with only the leftovers of the big players. However I'm quite okay with it at the moment, since it is better than nothing at all and the best thing for open source would probably be for companies to be a bit more liberal and not attach to their copyright forever, but open source it, when they made the most of their profits, which is what copyright was intended for initially anyway. Copyright was invented for book distribution in the very old days, where it took very long to distribute it around the world and it was only intended so the author can make a living out of his work and after that everything becomes public domain, but now as the time needed to distribute ones work is vastly reduced the copyright time should also be vastly reduced. This little change will probably benefit open source more than all the whining about ethics and morals.