As I was saying before the forums crashed... (-_-)
As with any other ban, yes - evidence would be needed. That's why I never mentioned anything about the system banning them.
However, this would eliminate the need to spectate multiple games or go through their game history manually. It would, essentially, speed up the process and free up the Moderators' time to do other things like handle Report Abuses, moderate the servers for other rule-breakers, and also leave free time for themselves.
From what I heard from a Moderator, the way that they deal with people who are suspected of boosting is they go through that player's individual games and see if there was anyone who helped them boost. Or if that isn't sufficient, then they have to spectate the person's games. This would automate the process, freeing them up to do other things.
As you may know, MCGamer has been trying to find or create new ways to prevent rules from being broken.
Starting with the standard Anti-Cheat to block Reach, Fly, and Waterwalking, among other hacks, and then installing the Anti-BSM system. I've heard rumors that they're starting an Anti-Forcefield/Aimbot system similar to an idea suggested by
Saimo, and if implemented this would be yet another way to make the process more efficient!
Edit:
Also, concerning the playing with bad friends part - If the "/boost game watch [playername]" tracked the remaining health of the two players, then if the last person standing had, say, 3 hearts left, then the system could conclude that they actually did fight. In addition, it could track
when the third and second player died, to see if the people had time to regen, escape, etc.