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I will use compress menu item with attached submenu as an example. Currently it contains compress to zip and compress to tar.gz items.

In my opinion, in case of items containing subitems, clicking on parent (items I describe) should invoke last sub-action or ran related application with selected file path. This allow to work faster and more intuitive.

Another example:
Copy into submenu of file in dolphin. When clicked, out normally copy item, allowing to paste it.

Problem would be to associate default action with submenu, because they're could exist many-many relation, but that is UX designer task.