This is purely theoretical, I doubt that practically any teams can run a 6-man roster properly.
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The 6th man must be an agent/map specialist. If a player has a limited, carved out role, then there is no practice overlap with what they other person is practicing, say the 5th man is a bind/split/haven specialist, and the 6th is for all the other 4 maps, then they won’t have to devote extra practice time to get them both reps on each map.
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Clearly defined roles. Every player must have a clear role on the team, so that when the 6th is subbed in and out there is no changing of roles and having to adapt your play style to accommodate the new player. A 6th man who’s job it is to come in and fill is a doomed endeavor.
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Fluent speakers. Inherently, a 6 man roster will have a harder time building chemistry on the server than a 5 man roster would, and having any type of a language barrier would just expound this fact more.
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Emotionally stable players. If you have players who tilt hard as hell, or your sixth is a nervous teenager on the stage then the pressure to perform on the sub could easily become very heavy, leading to dips in performance. All 6 players can’t tilt and blame their teammates for mistakes.