While I do agree that they look better without the border, it was added to handle cases where the user rotated their slide to be at a 90° angle to the viewport. E.g.:
![screen shot 2013-09-17 at 2 58 48 pm](https://f.cloud.github.com/assets/1009003/1159737/7280d8d2-1fcb-11e3-8d16-fcb1bd5856c8.png)

Slide 1 is rotated 90° about the Y-axis and becomes invisible.  I'm open to other ideas.