The following picture says it all:
Why do I want this feature ?
As of today, there are 87 sites on Stack Exchange. Each of us has certain centers of interest and making the list shorter will make it faster to browse. Time is precious when it comes to fine-grained "multi-tasking". If I have 10 seconds to take a look at something between the start and end of a compilation or of a file copy, I'd rather spend 1 second finding my sub Stack Exchange site of choice rather than 2.
There definitely are sub Stack Exchange sites that I will never dive into: With all due respect, I neither use Drupal nor do I speak Chinese. I thus obviously do NOT need them to appear in the list.