The of is required after couple in such phrases as a couple of items or a couple of people or a couple of hundred. Couple in those phrases operates as a noun, not an adjective. It is akin to pair (a noun) rather than several or few (adjectives). It therefore requires the preposition of to link it to the following noun (items, people, hundred). You would never think of saying a pair items; it makes no more sense to say a couple items.
Though even some good writers seem unaware of the proper idiomatic usage, we must not condone their sins of omission. From time to time, I stop reading an otherwise interesting book or article if the author reveals his or her ignorance by repeatedly failing to include the required of. At a minimum, I silently add the missing of each time I encounter a naked couple. It seems like the decent thing to do.