Joe,

I'm not an expert by any means. The pulley keeps the cables parallel. If you think about a servo arm, the cables will actually get slightly closer when you go off of center. The pulley is a circle, so the cables remain the same distance apart. The pulley keeps the cable that's in tension as well as the one that's not tight. When it's setup right, there aren't any issues. It's also a conversation starter for someone looking in your airplane.