I first wantr to state that I like this logo. Now, as for why, a logo in my oppinion must look good in color or B&W. It must look good at any size. It also must provide a good first impression.
This logo looks great small, large, B&W and color. It also has a professional look like the logos from Lucent, SGI and SUN. This logo to me speaks that this is a serious company that is here for buisness so people can rely on Debian.
The two logos are also important as you can have an official logo that you can control where it is used without keeping people from using the logo freely. You can see it as a Debian certified logo and a Debian compatible or Should-work-on-debian logo.