Melee Scoring and Aggressive Movement
I'm initially skeptical it would be advantageous. Against the weaker bots, you'd only be able to eek out a small amount of extra score, while against the stronger bots you're temporarily teaming up with, you have a lot of room to gain points by killing them instead. (E.g., going from 50% to 55% is a lot easier than going from 94.9% to 99.9%.)
But upon further thought, if two top bots already know they'll get about 50% against each other whether they team up or not, why not save that battle until the end and maximize the score vs everyone else by killing them all first. I don't really see it ever happening, but maybe it could be viable.
Edit: Using the terms from the Wikipedia article, I'm trying to say that I'm not sure R > P. You may have relatively little to gain by maximizing score vs the weak bots, and you have much to gain by improving score vs the other strong bot. For the weaker bot, of course it's advantageous to team up, but it may never be for the stronger bot. (I'm also presuming that any bots trying this would be the strongest bots, which I think is likely.) (And of course this is all complicated by differing performance in Melee vs 1v1, too - teaming up with Diamond and then facing him 1v1 every round might be a bad idea.)