You've been here since 2002 and don't know what a WAC is?, a WAC, for want of a better description, is basically a plugin.
Winamp3 was basically built with a variety of different WAC's , one for the playlist, one for the media library and so on, the theory was you could extend the functionallity of Winamp3 by adding or subtracting whatever WAC you wanted to, without having to do much to the main core of the application, in this case the core of the app was WASABI.
Because winamp5 was built on the old codebase of winamp2, WAC's were left in the past, one or two of them you can use because of gen_ff.dll, jpeg loader for one (gen_ff.dll is basically WASABI compiled in such a way it can be used with the old WA2 codebase).
Something like that anyhow.
Try the old wasabi development forum for more information..
In respect of changing gen_ff.dll, the only concern I would have with that is if you fix something that then subsequently breaks something else.
Look how many times scripts had to be fixed/changed/recompiled each time a new build of Winamp3 came out, even when 5 was released and gen_ff.dll was getting changed between releases, things got fixed while at the same time breaking other stuff, I would hate to have to go back and start revisting old skins AGAIN, having changed some of mine 8, 9 or 10 times in the past.