I agree with grading according to skill. I've seen people who have had the days fail because of skill. And I don't like the mentality that you should be given a belt just because your days are up. Frankly, it takes most people more than 600 days to be good enough to meet the standards of Nidan.
But I disagree that keeping track of how long you've done something is useless and just for money. I see where it has its place in correlation to a student's development. It is generally accurate. It does take about 60 days or more of training for most people to be able to pull off a 5th kyu exam.(according to the standards set by this particular federation.) And it does generally take 700 days or more after nidan for a person to be able to perform up to the high standards of a sandan exam.
Hombu dojo goes by days as well for this reason.
I've seen someone get shodan in less than 3 years - but he really was a "natural." I've also seen people sit at sankyu for 6 years - oh, right, that was me. (helps to have a dojo nearby, which has training sessions after, rather than during, work hours)
When I tested for Ikkyu, the two others and who also tested were told by the shihan that he wanted us to test Shodan within a year - I did mine in 13 months. Almost 3 years, a country and two dojo later, I went for nidan, but IIRC the requirement was minimum 200 hours and minimum 2 years.
It's still all a mystery - If I don't get confused at some point during an aikido practice, I don't feel as if I'm learning..