This might be revisited if future content results in weird situations, as some of the cases are kind of borderline (eg: Rituals of the Destroying Flame), but for now the simplicity of "always" wins out.
Element Markers placed on the board for Elemental Invocation are not considered "pieces" - the first printings of Jagged Earth considered them as pieces that were basically non-interactable (since they're not in any land); it's cleaner to just not count them as pieces at all.
Tags: Element markers, Pieces, Changed rulings, Despicable Theft, 9 more...