There's tons of cheating even in server-based systems. Just have a look at how every RTS ever has been filled with hackers.
Servers are a great tool to filter out gross cheating, but it can't prevent people from doing stuff that is unfair, simply because servers still have to communicate with the game clients and synchronize them, and while doing so the Clients will inevitably have to be sent information that should not be available to other players.
That's of course a bit less of a problem in Turn-based systems than it is in real-time settings, but servers don't just solve cheating. Especially not if the whole framework of how online play works was not developed to be server-based in the first place.
Ryika please do not confuse shared state multiplayer with server state. The reason cheating is rampant in RTS like Warcraft 3, Starcraft 2 and etc is because they use shared state, meaning every player has a copy of the game state, and the game state is then kept in sync by the players, if a certain player falls behind, all the players freeze up and wait for the droppped/lagging player.
Blizzard made Starcraft 2 at an awkward time. I guarantee you Starcraft 3 / Warcraft 4 will be server state, there will be no more maphack.
Games like League use server state, where the game client needs to request from the server any action, so by the client not having a copy of the game state, the client cannot know where other players are on the map and basically cannot know anything it is not allowed to know.
Applied to World of Warcraft, if WoW used shared state and you were in a raid. If Leeroy started lagging and disconnected, the entire raid all mobs and all players would freeze and wait for Leeroy. If Leeroy did not come back in 45 seconds the players can drop him, unfreeze everything and keep playing.
With server state, if Leeroy started lagged and disconnected, only Leeroy would be affected. Everyone else can keep doing the raid until he comes back.