Cancelling nasty treacheries is extremely powerful, especially for free, and especially especially when the only consequence is you having to a bit more of something you're probably specifically built around doing. Hell, if you're running Milan Money or similar incentives to investigate, you might even "break even" in terms of the action cost of putting down the clue.
It strikes me as particularly good for Joe Diamond in the main deck, as his big weakness is treacheries.