Agreed, it's a stupid rule. If you pay for F (miles or cash) I think you should be able clear standby into F. Unfortunately, the procedure is to process upgrades first, then standby. As a result it's a moot point since by the time they process standby, there won't be any F seats available. I have been successful once at getting upgraded to F after clearing standby (whole flight was wide open) but it took an enormous amount of typing and I did not endear myself to the agent.