There's no doubt that accumulating data on diverting asteroids is a good thing, but we also know that these bodies vary enormously in their distribution, composition and structure, everything from rigid bodies of solid nickel-iron to loosely aggregated rubble piles and fluffy snowballs. Whatever we learn from DART may just not apply to any other potentially hazardous body.

I would suggest that the resources devoted to this mission would be better employed in a general survey program of the asteroid belt. We need to know more about the asteroids themselves, how many of them are dangerous, which ones can be easily nudged out of orbit, than in how easy it is to move THIS particular body.

A general asteroid survey project would also yield strong scientific dividends, the Belt is a reservoir of primordial stuff, material left over from the formation of the solar system. What we learn there can tell us much about how planetary systems form, what is unique to ours and what it has in common with others.

I don't know much about this particular mission, perhaps Dart will collide with Didymos AFTER conducting a thorough survey. In that case, then I agree with your assessment. It is an efficient use of resources to piggyback the crash test with other studies. I agree planetary defense is important, but I believe that cause can best be served by learning about the Belt as a whole, not just concentrating on one of its members.