dude I can link you to
so many authoritative talks and articles that will basically confirm what darkshark and others here have said.
"irrelevant mechanics" are part of what games are because
games themselves are irrelevant. (I don't know what you mean by 'irrelevant', because I don't know how a mechanic can be relevant, so for this I assume you mean arbitrary.)
Having a direct measurement of a skill that progresses over time mixed with a linear system of achievement is a recipe for a dead game. Seriously. You can have skill-based achievements but you also need achievements that aren't based on skill, because when you test a skill that's so cut-and-dry as your grade on a song you end up with a semipermanent aristocracy of achievement, and no new player finds that attractive.
Here's the scenario when a new player plays KBO and all they have to work with is the grade on a song:
"well, shit. I'm 150th out of 160 on this song. everyone has the max score, let me try a harder one."
"oh wow, I barely got 20 notes into that. this guy almost full comboed the song."
one week later
"hmm. I need a game to play. What about KBO?"
"...nah. I don't have anything to do there except be good, and those guys are already way better than me. when I get closer to their position, they'll be even better than they are now. why bother?"
if you STILL don't believe me, watch this talk:
http://www.ted.com/talks/tom_chatfie...the_brain.html
It's tempting to think "well, we had a lot of people on KBO before, but now no one wants to participate." KBO's original success was partly because it had FFR's success to work with. And FFR succeeded in part because it made everyone feel like their efforts would actually do something, even if it was an arbitrary something. If you have a grandtotal, sure it's not a good indicator of your skill but people feel like they're actually getting somewhere. They have room to move. If you play only for better grades on songs, things are going to get stagnant really fast. Sure, some people might be motivated, but the vast majority won't.
This doesn't just apply to music games; it's a principle which holds across all kinds of volunteering and is why political efforts at the state level work
way better than at the national level: it's easier to be involved when you feel like your efforts are doing something.