I didn't want to state it again (which is why I didn't post this earlier), but I might as well do it now. My personal opinion about VGM7z is that it's a very bad idea for most purposes. True, it reduces the size of the global archive, but it introduces an unneeded layer of complexity for the player, and the individual files are much bigger than they should once they're extracted. For the same reason, I don't like the RSN and RGM "formats" either, since they're only RAR files in a wig (in fact, I always rename them to .RAR and use them as such); as a matter of fact, a quick research on Google further proves my point (
some guy having problems with RGM files on the Winamp forum).
As of now, only smspower officially supports (and encourages) VGM7z; we don't, and Project2612 afaik still provides only the ZIP files, even though they are
considering the possibility to provide VGM7z files too.
Furthermore: everyone and their moms know what a ZIP file is and how to handle it, while 7z is still relatively obscure to the occasional guy. Even though the ZIP compression is
much worse than RAR and 7z, it is one of our goals to be as cross compatible as possible, across every operating system. In fact, I can tell you something. Windows 7's built-in ZIP handler is broken (unsurprisingly, like 99% of the things by Microsoft), and it refused to open one of the ZIP packs I uploaded on this website (even though every reasonable ZIP manager, including the
1993 version of pkunzip for ms-dos, could open it just fine): ValleyBell recompressed it with a different program, and the ZIP file is now slightly bigger, but those people who (for some reason) use Windows 7 can now open it too.
tl;dr reducing the file size is good but increasing the compatibility is better.