OK, I may not be technologically competent to address this, but if the objective is to facilitate a computer-aided search of emigration from Canada to the USA, would that particular purpose not be served just as well by the sentence in the biographical narrative, "In 1924 John Smith emigrated from Vancouver, British Columbia, Canada to Vancouver, Washington, USA" compared to [[Category: Emigration: British Columbia, Canada to Washington, USA]]? And if the category is there, then there MUST be something in the narrative with a source which confirms the fact that the category testifies to. Otherwise, the category should be removed.
The functional purpose of categories is to group profiles. It seems to me that it is a misuse of categories to employ them to simply identify categories, when no purpose is served by grouping them.