I work on about 20+ projects, and my emulator is the only one that is GPLv3 licensed. The rest are all ISC licensed. (Note that parts of higan are ISC licensed, and I have granted other emulator authors exemptions, eg Snes9X now uses my audio core.)
The reason in higan's case, is because of the culture of the emulation community. There's a long, dark history of closed source emulators (even now with SuperGNES, no$sns and SNESGT), and I feel that emulation is a bit different from ordinary software: here, we are working under the threat of time. We realistically only have another 10-20 years to improve SNES emulation before finding a working system and persons interested in reverse engineering it are impractical.
I think it's very shallow and narcissistic to take the research of others, and not contribute back your own findings. I view this as more of an academic / historical preservation project than a regular desktop application. To end users, it's just for playing games, and not tied to any other system components, so I feel less bad about it.
To give a real world example, I encountered a bug in SuperFX (GSU1) emulation, and found that FuSuYa had a tracer-based fork of Snes9X: he basically added in a few fprintf() statements, and then refused to release the source to his changes. That would have been fine, but he botched something and introduced a very serious bug, one that I was also experiencing in my emulator. If I were able to diff his source to the official Snes9X source, I would have found and been able to fix that bug in a few minutes. Instead, it took me about two weeks to track it down.
Once I feel that SNES emulation has reached its limits in terms of research, I'll likely release the emulator under an ISC license as well.
Now I'm sure you can view this as me being hypocritical, and maybe it is, but that's my answer: I feel there's something more important than licensing at stake here.
The reason in higan's case, is because of the culture of the emulation community. There's a long, dark history of closed source emulators (even now with SuperGNES, no$sns and SNESGT), and I feel that emulation is a bit different from ordinary software: here, we are working under the threat of time. We realistically only have another 10-20 years to improve SNES emulation before finding a working system and persons interested in reverse engineering it are impractical.
I think it's very shallow and narcissistic to take the research of others, and not contribute back your own findings. I view this as more of an academic / historical preservation project than a regular desktop application. To end users, it's just for playing games, and not tied to any other system components, so I feel less bad about it.
To give a real world example, I encountered a bug in SuperFX (GSU1) emulation, and found that FuSuYa had a tracer-based fork of Snes9X: he basically added in a few fprintf() statements, and then refused to release the source to his changes. That would have been fine, but he botched something and introduced a very serious bug, one that I was also experiencing in my emulator. If I were able to diff his source to the official Snes9X source, I would have found and been able to fix that bug in a few minutes. Instead, it took me about two weeks to track it down.
Once I feel that SNES emulation has reached its limits in terms of research, I'll likely release the emulator under an ISC license as well.
Now I'm sure you can view this as me being hypocritical, and maybe it is, but that's my answer: I feel there's something more important than licensing at stake here.