I think a passport in a BOB is a legitimate use, much more so than say, EDC'ing a passport "just in case". Your BOB is when something "bad" has happened and you know that you have to get out of Dodge. Depending on the likely disasters in your locale, your home may not be there when you get back or may be inaccessible for a long time, so it makes sense to take important documents with you. And, if you're near a border and there's a good chance you'd cross over (say, you had friends/family on the other side that you could stay with) then certainly a passport is vital in that situation.

That said, if your BOB is kept stored in your vehicle or other place with a higher risk or vulnerability to theft than your own home (I'm talking about before you need the BOB, not while you're actually bugging out), I'd have to think twice about keeping valuable documents in the BOB. I'm thinking more along the lines of a BOB kept at home and sensitive documents are stored securely and then put into the BOB just before I need to head out.