How smoothly do you think your raid would run without Vent? The instant connection of voice communications makes short work of strategizing before an encounter. It allows raid leaders to direct the flow during battle, issuing new orders, calling out phases, and reminding players when to move. It allows for a certain amount of friendly banter during lighter moments, and it unites the entire team in whoops and cheers when the boss finally goes down.
The players of Durus Veritas make their way through the heroic raids of Mists without any of that. In fact, most members are partially or completely deaf and can't hear anything in the game at all. Even its hearing members hear nothing, because the guild eschews voice communications. Despite that, the guild is now 8/13 in heroic Throne of Thunder, chewing steadily through every barrier and demonstrating that fellowship, like raid progression, depends on clear communication -- no matter what form.