I would say it's useful as just another convenience for the customer. And customer convenience is usually a GOOD thing!

I would suspect that many customers do not even notice that when they click on "Continue Shopping" that it just minimizes the window, not close it! So, if they want to view what is currently in their cart; they may feel like they can only view the cart by clicking the add to cart button again!
That might scare some timid online shoppers away!