Under a deployment of a Lyceum community, I can see it as very desirable to have the ability for the site admin to define and then lock the categories to be used for classifying posts. A good example of this is that, if I were making a site that hosted 1,000 cooking blogs, I'd most likely want to enforce such categories as "Gadgets," "Recipes," "Exotic Ingredients," etc.
Other than that this helps to create a standard organization across all user's content if that is the admin's goal, but it enables the landing page to cull content from many of the blogs easily. Thus, if I wanted to have a spot on my landing page that showed the 10 most recent "gadgets" entries out of the 1000 blogs, I could. Without fixed taxonomy, we would be limited to harnessing all the content with a mere "most recent posts" box that would show any content type.
SOO, while you're in the code, can I also note that this idea brings about the need to create site-wide RSS feeds by category. So, just as I could have a "recipe" feed from User X, I think we should be able to create a cookingblogs.com "recipe" feed that would ding every time any one of the 1000 users posts something under "recipe" category.