I sort of agree with AMD Junkie on the original question. Grades are a measure of effort for most people, but more importantly, it's a way to encourage students to develop the habit of studying.
I was a pretty good student in high school. I mostly did my work, but it was rather effortless. As a result, I had a horrible studying habit, which came back to bite me HARD in college the past couple of years. During many nights before the exam, I wished I had listened to my high school teacher about not cramming materials the night before. I mean, I can still change, but I bet it'd be a lot easier had I developed that habit in high school rather than starting over now.
Another thing I wanted to point out. Several people mentioned being a computer junkie and grades weren't important. Yeah sure, to be a computer junkie, you don't really need to know anything about European History. But IMHO, a person educated in different fields is more likely to provide useful inputs to society as a whole than those who just buried themselves in their own field of study. A perfect example is the intelligent inputs in some of the debates this forum has been having about political and moral issues. Most of the authors of those intelligent posts are not only proficient in techie stuff, but also on other issues that affect the society. That's what makes them so respected.
So I do think grades measure the effort, but it's also important to know that why grades are measuring effort instead of intelligence.
just my 0.02