Harner was obviously aware of the solid support E..C. Colwell's article on John 1:1 was getting from mainly trinitarian advocates for decades, and how the NWT rendering of John 1:1c was being constanly brought up as an example of "un-professional" translation. Harner saw further.
He knew Colwell was not fully right in his conclusions, and at the same time did not want to produce any scholarly material that would support the NWT. Hence, his unwillingness to classify the structure of John 1:1c as indefinite. He took a middle road approach... the qualitative force of anarthrous nouns.
Though I agree with much of what Harner's study advanced, I also see Beduhn's consideration on John 1:1 as insightful and bold. The problem I see with those advancing the qualitative force of anarthrous nouns as in John 1:1c suggesting just quality and not indefiniteness in the classification, is that many a times the two are blurred in meaning. John 1:14 was mentioned: "the Word was made flesh." Certainly, the Word was not made THE flesh, since the noun lacks the article. But, the verse can also be rendered: "the Word became human", or "the Word became a human being."
The same happens with John 4:19 where the woman speaks to Jesus and says: "I perceive you are a prophet." The sense is best described as indefinite-qualitative. This is why some foreign language translations render John 4:19 as: "I perceive you are prophet," while many others have it: "I perceive you are a prophet."
I believe John 4:19 is similar to John 1:1c in that both anarthrous nouns preceding the verb lack the article, and definiteness is certainly not the issue, but whether the emphasis falls on indefiniteness or qualitativness depends on the context and the translator's choice of rendering. The Trinity doctrine is a spoiler in the exegesis of John 1:1c.