It's a tough tradeoff for society, and a lot of harm is concentrated, but in a way that's good thing - there is a way to block tor exit nodes if you need to and the defaults ports do prevent many types of abuse and since the exit nodes are public they can just be blocked for spam, clickfraud, etc. But with any duel use technology, the opposite argument would be investigations also running through TOR, or even a totalitarian state (in this case it seems non technical judicial procedure through proper channels, but that's the concern). The trouble is the routing is outside of the state control and the typical mechanism for takedowns, ultimately for the worse of the worst ultimately has a host somewhere else as tor just does the routing. Since by design the exit node wouldn't necessarily get you any further up the chain to the middle node in the connection, it would be more fruitful to chose a different investigative strategy.