Well, they are useful up to a certain extent, nearly as useful as repeated standing vertical jumps, but with less of a training effect due to the encouragement of incomplete hip extension to hurry and pull the feet up, and the extremely low eccentric landing forces. You can watch video after video of people performing high box jumps, and more times than not, they are pulling the knees up before they reach complete triple extension. But if someone were to want to work on rfd with low impact forces, teach landing skills at a low level, etc. they can be beneficial. They are not a valid test of jumping ability in any way, and very low eccentric forces involved make them a lower intensity training technique.
The issue I have with box jumps is when people are jumping on extremely high boxes, believing they are training their jumping ability, when in reality they could cut the box height in half, step off it and do a depth jump, and get 100000000000 times the training effect they are looking for.
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