I feel that this would remove certain levels of diplomacy. If I ally with someone, they might not trust me if I don't give them vision. And I mean, maybe they shouldn't trust me, but that's part of the game. They have to rely on word of mouth. With vision implemented it would make misinformation almost impossible. Right now, if I lie about something, and someone else says I'm lying, you can't trust either players, because either of them could be lying. With vision, this situation wouldn't be very common. It would also make specialists like the intelligence officer and the princess less useful.
"You want to believe that there’s one relationship in life that’s beyond betrayal. A relationship that’s beyond that kind of hurt. And there isn’t."
-Caleb Carr