Cannot Sign In to enable iTunes & App Store within iOS Simulator
In-app purchases would be the only reason you need to sign into iTunes/App Store since you cannot download apps from the App Store in the simulator. 

According to Technical Note TN2413 you cannot test in-app purchases in the simulator.


  Cannot connect to iTunes Store
  
  The "Cannot connect to iTunes Store" issue may be due to one or more of the following reasons:
  
  
  The sandbox may be unreachable.
  Your app does not have a bundle version (CFBundleVersion). See Setting the Version Number and Build String for more information.
  Your app is running in the Simulator, which does not support in-app purchase.
  You are attempting to purchase a product that is unavailable for sale. See Query the App Store for product information before presenting your app’s store UI for more information.