Open Borders generate trade routes, which over time can become very profitable, especially if you have a lot of coastal cities with harbors and custom house, I don't look at AI code and all that but I'd also assume a civ is less likely to goto war with you if your generating them 50gold/turn. They also increase relations with the civ you have them with.
The bad part is it decreases relationships with civs that don't like the civ your trading with, they may ask you to stop trading with them or yatta yatta yatta, it just makes them cranky.
I never consider open borders just unit passage, it's a trade pact to generate commerce between your two cities.