Should 'of' be a preposition?

This is an excerpt from Thomas Bloor & Meriel Bloor's, "The Functional Analysis of English: A Hallidayan Approach"
(Hodder Headline Group, London, 1995)

p. 147
Prepositional phrases with 'of'

We mentioned that of is the most frequently occurring preposition in English. This information comes from Sinclair (1991), who calculates that of occurs more than twice as often as any other preposition. Sinclair's observations, which are based on a massive collection of English text, the Cobuild corpus, challenge some of the standard grammatical descriptions. As we have seen, prepositional phrases realize two main functions: Adjunct in a clause and Postmodifier/Qualifier in a Nominal group. Sinclair points out that it is generally assumed that the most typical (that is, frequent) function of prepositional phrases as a whole is the Adjunct, and for most prepositions (in, on, up, and so on) this is true. However, he notes that although OF does, like other prepositions, show up with this function (for example: 'convict these people of negligence') such occurrences are relatively rare, and the overwhelming majority of phrases of OF are Postmodifiers. He also notes that, unlike most prepositions, OF has no basic spatial sense (of direction or position); compare UP, ON, IN, OVER, UNDER. On grounds of distribution and typicality, Sinclair goes on to suggest that perhaps OF should not be classified as a preposition at all, but belongs to a class of its own. To date, this position has not gained widespread acceptance, but the argument is a powerful one.