Of course, if you completely overlook RBN as "just a bunch of garage bands" (which is what ALL bands are, before they get famous, note) odds are you're going to miss out on some good songs you might have otherwise enjoyed playing... but hey, your call.
As for the original suggestion of just sharing the MIDI, that's not going to work. I tried doing something similar with free DDR style programs years ago, supplying charts while the user rips their own albums. Every rip is a little different in terms of timing; maybe they'll use the version off a Best Of CD instead of the original album, or maybe the ripper introduces minute time compression, etc. It never lines up properly and it's not worth it. Stick with FoF or a similar freeware engine and take your legal chances, I guess.