DoFs are a big deal. They are hard to get, you only have a few at a time, violating them has big consequences, you have to pick who you sign with carefully, so its ok for them to have a sizeable impact on economy. Open borders aren't like that. They're easy to get, you have have them with everyone, violating them has no long-term consequences, there is basically no reason not to have them with anyone you have contacted.
Couldn't open border agreements be made into a big deal like DoFs? Increase the AI's value on open borders, make it so that they will not agree to open borders unless they are happy with you and trust you. This would help make diplomacy more worthwhile. And I think there are more reasons right now to avoid open borders than to have them: militaristic civs will scout out your military (and I tend to under-invest in military, especially early in the game), and expansionists will march a settler through your territory to settle that spot you were saving until you finished a coliseum or two. I'm not saying the bonus for open borders would have to be huge (I think DoF should definitely be bigger than open borders); it could just be something like a trade route between the capitals (assuming they are connected). But having a good reason to accept these would go a long way.