When other factions swear a pact for the sake of licking your boots, that means that the game will see it as your victory. You don't need to worry about them as they will never bother you again (just don't let them get too powerful (or they'll forget your deal with them).
Besides, the extra vote in planetary council can really help you in becoming supreme leader (if you are after a diplomatic victory). I was once using the University. The Peacekeepers and Gaians both swore pacts quite willingly. The Morgans and the Spartans were eradicated, and we were all at war with the Hive and the Believers (who were at war with each other too). I forced Yang to surrender, thus making a pact with him. I pretty much repeated the process with Miriam. Because I had a pact with everyone left, I'd won a diplomatic victory.
Another example is when I was the Gaians, the Peacekeepers and University each had a pact with me. I forced Morgan to surrender and forced Miriam to surrender. Even though Yang and Santiago were against me, I still held the majority I needed to win the Supreme Leader election in Planetary Council.
And about the player surrendering to the AI... Its pointless. If I were forced to do so, I'd simply rebuild my hordes of mind worms and overthrow the oppressors whom I was forced to serve.