With all due respect, I don't know why this bothers bloggers. If a blog is good, like DUFU or Malkin's blog, I'll go there even if the address has "bop-shebop-shebop-rama-lama-ding-dong" in it, and if it sucks, I won't go to it even if the address is "ablog.com."
But if I want to print up cards or other material with the URL, "BlogSpot" takes up too much room. Also a lot of people type out the URL from memory. Oh, and then I would like to print up DUFU T-Shirts and want the URL as short as possible.