[wycats]

If you look at the second conditional comment, it will not be parsed as a comment in browsers without conditional comment support.

In other words, it will correctly use jQuery 2.0, like any other browser without conditional comment support.

[seldo]

It's a conditional conditional comment. I feel ill.

[WickyNilliams]

It's known as a downlevel revealed conditional comment. You will see it in use in the HTML5 boilerplate. That said, I echo your sentiment, it is an immensely horrid syntax. For more info: http://css-tricks.com/downlevel-hidden-downlevel-revealed/