1. Where else would you go to post something? I mean, like would you want all the posting options available on the publicly displayed page of your blog? I guess that would be possible, but posting (or editing) an entry is one of many "administrative" tasks, so doesn't it make sense to go to the admin area to get them done?

2. You can give posting permissions to anyone you choose to give permission to. The person registers, and then you as The Admin decide what permissions (if any) they get in which blogs. Permission control is really complex though because bloggers are both individuals and members of a group (you pick the group), and each can have permissions in each blog. So it's often confusing, but to answer your question: yeah you can allow a guest permission to post.