Software Update has no need to be in Applications - you're meant to access it through System Preferences anyway. As for Screen Sharing, it only makes sense to be available when there are networked computers visible that support it, and since it has hooks into the Finder and iChat, it doesn't really need to be openable by end users, unless you're trying to share with a computer that, for some reason, isn't visible on the network. Then, I can understand.