Recent discussion about ADOM's source code is asking for my opinion. If you really think the benefits of open source I can only think one and it's the ability to compile to another platform. People always say that open source is great because everyone can take part of development. I think it's the worst thought, because people per se are not good developers. Even if it is open source we need a great game designer (preferably a team leader) who knows what to do. There are not many people who know the stuff and most of them work in commercial game development.
Then maybe someone can fix bugs, yes? Not likely. Bugs are typically hard to find and/or fix and people seem to be reluctant to take part in that work. It's tedious and not fun.
Even when it's easy to change the game content it's less obvious that anything good will be produced. Is there any Angband variant better than the main game? I don't know.
The usual problem is however that the source code is really difficult to understand and it's even harder to make any modifications without breaking something elsewhere. Nethack is notorious for that. DCSS has been a kind of success, but it didn't happen easily when the developers continued from the original Crawl source code. I have seen it and I can imagine it was not an easy task to continue from the state where Linley left the source code.
Trying to work with open source people is difficult, because there are no rules or obligations for anyone. Good graphic artists are impossible to find to work free. It's also difficult to match another programmer in the development, at least someone who don't want to be a game designer like you. Two or more game designers is not going to work well.
Now that ADOM Deluxe is going to be a commercial game Biskup made a wise choice when he did not release the source code. He has eliminated the possibility for someone to make ADOM Deluxe Free based on ADOM's source code. I bet we will never see ADOM's source code open so please stop whining about it.