There's two sides to this coin.
1) If two players are ready to throw in the towel, the game is over. The chances of winning are low, morale is low, team dynamics are pretty much thrown in the trash. If the surrender vote fails, the summoner's code should hold those players to their best until the end of the game... but we all know that's not reality. Once you consciously decide to lose, you'll lose... unless the enemy makes an even harder attempt at doing so.
2) If two friends are playing a game together, you should have an advantage. They have an advantage, anyway. However, if they see you as a weak link, and decide to surrender even though the chances of winning are in your favor, game over. Furthermore, teams could abuse this. If I'm an ******* and I want to intentionally drop my Elo so I can get easier games, all I have to do is get one friend and start spamming surrenders... the third player has no say in the matter. This doesn't currently happen because it can't be done. Open that gate, and it will happen.
So, in my opinion, the negatives outweigh the positives. If you really want the third player to surrender, communicate with them. Explain that the enemy is super fed, and you just want to get to the next game - you'd appreciate if they would do the same. If they deny, well, it's your responsibility to play your best until the game is over. **** happens, and so does the opposite.
That being said, I would also support an earlier surrender time. Especially in the case that a player has gone AFK.