I think it is a complex question to decide whether to open borders with an AI.
First of all, an AI may wish to open borders with us for any of these reasons:
1/ Spread religion (If the AI has a holy city, then inevitably I will open borders with them. Getting a religion spread into my cities is almost always a good idea and it makes sure that I will not end up being locked out in AP. The one possible exception is where I intend to rush that AI because I do not want the "we do not want to fight borthers and sisters of same faith" unhappy face penalty in my cities. But then again, it is better for the target AI to be spamming missionaries as opposed to military units.)
2/ Scouting (We can this when an AI parks a scout / chariot outside your border and then asks for open borders, normally I will agree to open borders unless it is a warmonger AI who is cautious / annoyed with me.)
3/ Preparation for war (We see a large SoD roaming in a nearby city and the AI wishes to OB. Chances are that it wants to send its spies in to do some missions. OB is probably NOT a good idea. An exception is that it is already pleased / friendly (depending on which AI) with you and is NOT WHEOOHRN. If that is the case, it probably wants to go past your land to fight another AI. If so, OP is not a problem.)
4/ Trade (Some AIs want the trade routes, these tend to be the more peaceful ones. Sure thing, OP with them.)
5/ (This is only relevant late game) Sabotage missions / tech stealing (If I am aiming for space race victory and some cautious / annoyed AI with a large amount of EP wants to OB with me, obviously the answer is NO.)
The next question is when I want to OB.
1/ Suck up to an AI.
2/ Scout his lands to prepare for a rush.
3/ Religion spread.
4/ Running an EE.
I think, generally speaking, if you have a powerful army, be more generous with OB.
I do not believe it is wise to OB with every AI just for the diplomatic benefits. Sooner or later, an AI will demand you to stop trading with his worst enemies. So watch out who is whose worst enemies and make careful choices in my opinion.
First of all, an AI may wish to open borders with us for any of these reasons:
1/ Spread religion (If the AI has a holy city, then inevitably I will open borders with them. Getting a religion spread into my cities is almost always a good idea and it makes sure that I will not end up being locked out in AP. The one possible exception is where I intend to rush that AI because I do not want the "we do not want to fight borthers and sisters of same faith" unhappy face penalty in my cities. But then again, it is better for the target AI to be spamming missionaries as opposed to military units.)
2/ Scouting (We can this when an AI parks a scout / chariot outside your border and then asks for open borders, normally I will agree to open borders unless it is a warmonger AI who is cautious / annoyed with me.)
3/ Preparation for war (We see a large SoD roaming in a nearby city and the AI wishes to OB. Chances are that it wants to send its spies in to do some missions. OB is probably NOT a good idea. An exception is that it is already pleased / friendly (depending on which AI) with you and is NOT WHEOOHRN. If that is the case, it probably wants to go past your land to fight another AI. If so, OP is not a problem.)
4/ Trade (Some AIs want the trade routes, these tend to be the more peaceful ones. Sure thing, OP with them.)
5/ (This is only relevant late game) Sabotage missions / tech stealing (If I am aiming for space race victory and some cautious / annoyed AI with a large amount of EP wants to OB with me, obviously the answer is NO.)
The next question is when I want to OB.
1/ Suck up to an AI.
2/ Scout his lands to prepare for a rush.
3/ Religion spread.
4/ Running an EE.
I think, generally speaking, if you have a powerful army, be more generous with OB.
I do not believe it is wise to OB with every AI just for the diplomatic benefits. Sooner or later, an AI will demand you to stop trading with his worst enemies. So watch out who is whose worst enemies and make careful choices in my opinion.
