Challenges really are just beating the song again, though. It'd be different if, for example, that 20 second bit in 21st Century Schizoid Man was all that you played when doing the challenge, but you gotta go through the whole song front to back in order to do it. Fuck up and it's back to the beginning.
I take issue with the redundancy of something like that in an arcade style game like fake plastic instrument games. I won't even pretend that Harmonix has their fingers on the pulse for what gamers really want, but I'm not going to go apeshit over something like that when going for a high score is really all anyone could want.
Unless you've secretly been building an army of horrific GH create-a-characters with the parts that you've unlocked via the challenges, that is.