I agree that there is a high probability that a functional item that fits and flatters would be a favorite, but I'd be stretching to say that my black pencil skirt is a favorite. It fits, flatters, and is totally useful when I need to show up in that kind of attire, but I don't love it or get a particular thrill out of wearing it. Same goes for my plain black sweater, my winter boots, my J Crew navy peacoat, and my black pumps. They are all workhorses that get worn regularly, but are more like a steady, reliable backdrop for my favorites or serve a specific function that my lifestyle requires. Purging them would leave a hole in my closet, but I'd have no trouble finding a replacement that would work just as well.
My favorites, on the other hand, would be what I'd grab if a tornado or firestorm threatened--I'd be devastated to lose my leather moto jacket, my well-worn Gloverall duffle coat, my sleek black Chelsea boots, my perfect white T-shirt that I found after years of searching, or even my favorite, knock-around Blundstones.
ETA: I guess, to me, liking something for its functionality is different than liking something because I love to wear it. Focusing too much on my favorites in my recent purge made me forget that these functional, but unexciting, items also serve an important role in my closet.
