Well, not that it should undermine the accomplishments of Harmonix: they were constrained by legally licensing the songs *and* requiring full multi-tracks for every offering.
So I have *no* doubts that HMX would’ve got Zep if they could. Heck, I imagine ALL the great bands that they probably wanted but just couldn’t close the deal on due to lawyers or missing masters. Perspective due, providing customs is a different sort of game that has a lot more outright freedom behind it.
Not that I turn my nose up at the efforts of C3: I’ve been waiting for a group to draw a line in the sand and say, “We’re going to give you only the best.” I’m totally behind that mission statement, but I’m snobby and will probably confine myself to primarily releases with multi-tracks for all available charts! (or at least drums, heh…)