You miss one point, which is that GH3 has been out for for sometime and RB is going out now (could have used that time more wisely). Sure I could have bought GH3, but I didn't because I was waiting for RB, after the port debacle I am now waiting for GH4. And living in Europe waiting for these things to jump the Atlantic it changes your perspective. I only wait for the experience I want. The only thing I would miss out would be the drums, since I already use (and have great fun) a freeware software called UltraStar to sing songs I make myself.
Heck, I'm even looking into Ultimate Band and see how that turns out. I sometimes think these music games are too expensive for my wallet.