Y'know, I'm getting a bit tired of the "I don't understand reply ..." which was the response last year when I said multiple blogs was the one thing keeping me from making the move.
Consider the work of a charity, a school, a church ... subdomains for groups within the organization. So you get a main blog, then each group has its own blog - but all from the same blogging software.
On the front end, users might see:
http://www.mychurch.org - for general use
youth.mychurch.org - branded away from old fogies (e.g. different look, feel and content)
advent.mychurch.org - specializes in Christmas devotions
Each is branded differently, each has entirely different content, each supported by the same group of people - which is kinda hard to do w/categories - especially when categories become your points of menu navigation for each of the above.
On the back-end, one application/interface to rule them all.