A citation metric seems like a bad way of evaluating usefulness, but a good measure of another type of interestingness (are papers cited often in the next year likely to contain useful insights?)

To determine usefulness we need to look at something other than publications. We can hope to estimate how the state of modern theory affects modern practice---what ideas or modes of thinking are important, what techniques are used in practice, etc. Looking back, we then have some leverage to understand what research programs helped advance our understanding in a relevant way, or were indirectly necessary for the development of practically important techniques.