Five cool things about Mr. Rook:
His downside is secretly an upside. You draw your weaknesses for free alongside other cards rather than having to draw them later in place of another card. He grants you a significant amount of control over when you draw your weaknesses.
He is expendable. Unlike a certain professor of entomology, Mr. Rook works quickly. He is two health and sanity worth of soak, and if you spend a secret every turn you are happy to use up that soak two turns after you play him. He doesn't stop you putting other allies in your deck or force you to run Charisma. Which lead on to...
He is great with Calling in Favors. I had a game where I put Mr Rook into play four times and still had Mr Rook in my hand.
When you have nine or fewer cards left in your deck he lets you search the whole thing. It's like he hands you a No Stone Unturned (5) that costs zero at the start of the round.
When you draw a treachery with a skill test he can instantly search your deck for relevant icons. There is a player window in every skill test just before cards are committed.
Here's a bonus sixth thing: If your random weakness is Doomed you will live fast, die young, and start earning experience with your replacement investigator much sooner than you would otherwise.