I am a huge fan of mentorship; I benefited from it early in my career, and I strive to offer it to others now. I don't think that requires an open office at all, though. I think mentorship works even better if you're in a quiet environment where you can talk comfortably and not worry about bothering the people around you.

That said, I can attribute one positive effect in my career to having worked in an open office. Early in my career, I happened to overhear someone on the phone dealing with an Open Source licensing issue (and I genuinely do find Open Source licensing interesting), went over to chat with them afterwards, ended up finding out about the company's Open Source review process, one thing led to another, and not long afterwards I co-ran that process company-wide (for a 100k+ person company), which I really enjoyed and which provided a great deal of visibility into all the work happening in the company.

I don't think "happened to overhear one side of someone's phonecall and strike up a good conversation about it" is a "benefit" I'd say is worth the pain of an open office, but I still want to acknowledge it.