The list of sources that are currently tracked is actually selected from that list
Few things I skipped are:
1) Private/Beta/Unstable/Adult Oriented Sources... because I don't want to have people asking me why things doesn't work etc. when I'm not developing them.
2) Semi "known" sources, as we know, there have been two incidents (at least) where malicious packages are added to semi new sources branding itself as Jan software package, and 1.1.3 prep... Not something we want here Smaller sources won't be added until I add "anti-virus" like feature on here.
3) BAD malform xml sources / dead sources. You'd be surprised how many of the source plist doesn't follow the outlined formatting convention, or are simply dead. It's no wonder people with many sources have slow load time...

Over time, I'm looking into adding more sources later. If there is one that you really want to see, please by all means post it in the request thread some where on here, and we can review it as a community