When I add a ruler to a slide in a presentation - I don't want it added to _all_ slides of the presentation. If I wanted that, I would, say, expect to add it to the master slide. However - Impress seems to think that all slides need to have a shared set of rulers. That really frustrates users' typical intentions; and you can't even indicate you want a single-slide ruler, i.e. just the less-common user intent is assumed to be always valid.
Can you imagine users with a different POV? The ruler is a UI control like the slides sidebar or the notes pane - all toggled on/off independently from the document. => NAB