Here's my simple reasoning.
Cities: holds different information than states:, which holds different information than nations:, therefore, each is its own category. But, states/provinces are part of nations and cities are part of states/provinces, so their Page Tree includes several categories. I would like to use PagePath to select an Origin City, but they are parented to state, so, that'd be a no-go.

Being able to use an existing category for a field in a form is EXTREMELY useful. It would be nice to expand this functionality just slightly.

While I don't suggest allowing the creation of pages outside the initial category when entering information into a form (it seems like it would be messy and too much of a challenge to create a clean layout/script for it), if a PagePath uses an extant category which has items from another category parented to it, I suggest having an argument to allow pages from other categories to show up in the form definition.

For example:

  origin:
    label: Origin
    type: pagepath
    category: nations
    use-full-pagetrees: true

or, even more useful and powerful would be:

  origin:
    label: Origin
    type: pagepath
    category: nations
    show-other-categories: states cities