Up to now I assumed the only reason for registering a new account in WP without starting your own blog was so you can be a joint author for someone else’s. Playing about with my own WPMU installation reveals another possibility. If you register a new account as part of creating a new blog your blog name (the bit that ends up as part of the url) and your username will be the same. So if you create a new blog called testblog for instance and register as a new user your username will be testblog as well. However, once you have registered as a user you can create more new blogs and log into them with the same username and password (assuming the admin options allow this).

If, on registering a new account, you choose the create a user name only option and then log in to create a new blog you have bypassed the necessity of creating a blog with the same name as your username. Creating a new blog as an already registered user gives you frre choice of a blog name without it becoming a user name.

Once you have created more than your initial blog you have the option of changing the designation of your primary blog, by default the first blog you create. I assume if you do this it is possible to delete your original blog if you wish as your username is independent of any particular blog.