It has also got to be remembered that part of the reason colleges use the
SAT is because it's one more
piece of information they can use to choose between
students. Often colleges receive
thousands of applicants for just a few hundred, or few thousand, spots. Remember, not only are students vying for a
limited number of spots in the class, they are also often trying to get
scholarships.
I don't personally know how they choose who to admit and who to reject, but most of what I've heard and read is that standardized tests such as the SAT and ACT are a factor in their decision, but not the most important factor. They have to look at a variety of factors, including class rank and GPA, extracurricular activities, and essays. None of these alone would be a good indicator of how well any particular student will do in college, just as the SAT or ACT alone are not a perfect indicator. But when you consider all of these factors together, you begin to get an idea of what a particular student is like and whether or not they are what is right for a particular college.
College evaluators also tend to keep in mind that a poor SAT or ACT may only indicate that someone was not feeling at their best for three hours one saturday morning. A bad SAT does not necessarily reflect poorly on a student, but a good SAT does reflect well on a student. However, I for one am not entirely convinced that a good SAT indicates one will be successful in college.
I wouldn't be surprised if in some cases a good SAT score might actually cause one to succeed in college. For instance, say a student gets a good SAT score. After looking at all of the student's information, the college in question decides to grant the student a scholarship. Like many scholarships, this one requires the student to maintain at least a 3.0 GPA to keep the scholarship. Therefore, because the student happened to do particularly well one saturday morning, he now has much more motivation to succeed in college.
So is the SAT perfect? Probably not. However, the SAT helps give a college admissions board a little bit more information about a student, and they are seeking as much information as possible to help them decide which students to accept and which to reject.