Wikitree, like other genealogy sites, has enough problems built in, what with name mispellings, aristocratic titles of all sorts, lack of birth or death dates of any kind, and the like.
Putting someone in a category gives the person who visits Wikitree another link in their attempt to find a particular person. Putting someone in multiple categories gives the visitor MULTIPLE links to the person, any one of which might give them the connection they are looking for.
As I read some of these posts, I feel like I've been putting way too much time into adding categories to profiles. But, then again, I verfied which country the person came from, which region within the country was home, which town they were born or died in, maybe which cemetery they are buried in and, if it's a foreign country, I probably also have the same categories with names in the foreign language, as well.
Personally, I can't ask for anything better than a variety of ways to compare and contrast profiles to find the one I need. I have more problems with profiles with little or no information in them, other than a name, which may or may not be correctly spelled.
I'm also more concerned with duplicate categories, justifiably or not (example - Category:Bavaria, Category:Bayern, Category:Bavaria, Germany, Category:Bayern, Deutschland), not to mention higher level categories like Category:Germany and Category:Deutschland.
As I move around Wikitree, I see far more profiles with NO categories than I see with TOO MANY categories. I haven't added a profile to Wikitree in weeks, but I have plenty to do "tweaking" the ones I have, and categories is just one tweak I can make.