As much as I enjoy teaming with my friends, one very effective strategy is getting between two teammates and making them hit each other. It's one way that I used to take out teams of two. Also, there's boosting your teammates with fishing rods, punching them out of holes, etc.
In the Hunger Games, people constantly have to be wary to make sure they don't accidentally risk hurting their teammate. This would remove that aspect, and would make people have less of a need to be careful.
Also, what about backstabbing? If you pretend to team with someone, they might ask you to Truce with them. Then you can't backstab them! And if you un-truce, if it shows a message then they'll know right away, or if they see you hitting them then they won't even hesitate to think "What if they're just joking around?" And it would make backstabbing much harder.
jefeperro would disapprove. (ahaha lol)
It would encourage teaming. While teaming may not be a bad thing, there are a lot of people who really do not like teaming, and it would just further serve to drive those people away from MCSG.
Clans and Teams. It would change the way clan battles work. Many people have strategies that rely on confusing their opponents and making them hesitant to hit for fear of hurting their teammates.
When people are at low HP, they have to communicate. There's a few too many times when I've accidentally killed a teammate by hitting them a few times as a joke because I didn't know they were low. This would make mistakes such as this impossible, even though it's part of the game.
And lastly, unfair situations. Deathmatch teams against that one lonely person. They should have a chance to outplay to the best of their abilities, not be brutally murdered by two people who can't hurt each other.
All in all, I don't think this should be implemented. I do see some of the reasons behind this, but the pros cannot overcome the cons.