The display ad builder is a very useful tool for anyone who needs banners completed quickly, as you can see from the above video, the time required is far superior than starting a new banner in some graphics software.
I also prefer it as after many tests against banners created by designers, they tend to perform with a higher click through rate and have a lower cost per click. I am sure there are exceptions, but if you have a cost effective ad which often performs better than what can be designed, I would always give that at least a test!