"Use Cases" appendix is out of date and unnecessary

The "Use Cases" appendix is out of date and unnecessary. The diagrams depict PDPs that do not agree with what Section 4 says, etc. Back when the spec was in a more unfinished state, describing the use cases was a helpful means of getting readers onto the same page about what we are doing. However, the recent improvements in Sections 2, 3, and 4 have rendered this appendix unnecessary.