In a world where we had precision tools that could be used to deliberately alter brain morphology, the direction of causality would be highly relevant for trying to treat ADHD -- it would tell us whether there is any point in trying to use those tools. But for diagnosis only, I don't think the direction is actually relevant. That is, if we see a strong correlation between ADHD behaviour (as measured by, say, a standardised written test) and certain brain morphologies, then we're justified in concluding that ADHD has some objective physical manifestation -- that is, that the written tests aren't just measuring arbitrary collections of symptoms, they're estimating "something real". There might still be many different underlying causes.
Would this be useful? I think so: It would then be possible to assess the accuracy of a written test.
I think the risk of conflation with stimulant use that another poster mentioned is very real, though, and that problem is a close cousin of the "direction of causality" problem.
This is true and quite an interesting and nuanced point. But the connotation of this discussion, and something I expect applies to the overwhelming majority of people, is an association of physical manifestation (brain morphometrics or whatever) causes issue, rather than issue causes physical manifestation.
In particular this whole thread of discussion started back with somebody expressing a bit of skepticism about the definitive nature of ADHD (and other psychiatric disorders) and somebody responded with the brain morphometrics as proof of such, yet in reality that's mostly still just begging the question.
Would this be useful? I think so: It would then be possible to assess the accuracy of a written test.
I think the risk of conflation with stimulant use that another poster mentioned is very real, though, and that problem is a close cousin of the "direction of causality" problem.