Options "Keep past HTML reports" and "Allow missing report" should allow to display older report on main job page when new report is missing. It's very needed in case of configurable build which allows to rebuild partially only chosen functionality with it's report. In such case all missing reports are removed from job page instead of showing older ones.

I think that they should be removed or not based on "Always link to last build". If you want link to last build only, then they will be missing. Otherwise they should display older reports if no new is available.