Kathy, I thoroughly agree on the value of migration ship categories. There were many bonds among fellow passengers. Sometimes they were bonded before they emigrated (people from the same village traveled together), but many more bonds (including marriages) formed on the ships.
There's a history of some WikiTreers adding categories in lieu of biographical content in the profile, or seeking categories for commonalities that don't have any apparent value for grouping people together (an example of this might be "People Born in February"). Because of that, when I comment in about categories I usually mention something about the appropriate purposes for categories.