Yes, if LE has other evidence linking Alice to criminal activity (and Tor use), then verifying that a person of interest uses Tor is quite damaging. That was the attack used on Hammond. OP's question was more about the feasibility of performing a "fishing expedition" to narrow tens thousands of suspects down to the criminals. I believe that is impractical. How many people visit their PO box a few times a week? How many people use Tor for non-drug activities? The cost-benefit ratio of this attack is very high.