I think that this review in Slate, by Emily Bazelon, of three pertinent books (Bennett & Kalish; Kohn; Cooper) on the subject of homework tells it fairly simply:

"Cooper is one of Kohn's main foils and a leading scholar on the subject, so I picked up his book expecting to find a convincing counterargument defending homework. I didn't. Cooper's research shows that, much of the time, take-home assignments in elementary school are an act of faith. No one really knows whether all those math sheets and spelling drills add up to anything. If there's little or no evidence that younger students benefit from homework, why assign it at all? Or, to adopt Kohn's less extreme position in The Homework Myth, why make homework the rule rather than the rare and thought-through exception?"