Why did Zero/Vergil take over over Wesker?
Because unlike Wesker, they can wipe you out with basically no counterplay. They also have the advantage of being able to attack the opposite top corner, which Wesker just can't do barring random Phantom Dances.
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Twostuck, Homestuck RPG: http://www.gamefaqs.com/boards/8-gamefaqs-contests/62168402