... if racks are not used and tiles are visible, but isn't making your own 'runs' hard enough than to worry about what others can see?
I can see this as possibly valid if and when all players have many(lots of) tiles that they must still play to win. But lets say that a player has only one tile left to play, and if the other players can see this tile, then the other players could make sure that they dont create a set or add to an existing set, in where on the next turn, the player with only one tile could then play it and win.
Now if the sole tile was hidden, then the other players would not know what is was, and thus would possibly inadvertently "create" a set that would then allow the player to win on his next turn.
So yes, it is important that the tiles be hidden from view from all other players during your game.