The challenge with the one weblog and categories approach is that it forces you to use one fieldset for your whole site. With multiple weblogs you can customize the fieldset to be more appropriate for the content type being displayed.
Plus - if you have multiple content administrators you can setup member groups and permissions more specifically if you’ve used multiple weblogs.
I also like having Publish and Edit options that—for the most part - match the main navigation of the site. Clients seem to pick it up easier.
You can always use categories or relationships to pull content from seperate weblogs back together if need be.
FWIW - I have 19 weblogs on my Boyink.com install - running Boyink.com proper, a family extranet, and powering the site that is built in the tutorial series.