Strictly from how he could have improved the movie and the plot.
They could have used his explanations to further flesh out the Trogs during the journey. And the movie could have used one more kill before Brooder died.
I agree that he would have been interesting to include, but I don't think it would have driven the plot forward. He would have been shoe-horned in to the dynamic because he wasn't developed initially. He would have to have been introduced earlier so that you have character backstory or you'd have to get to know him on the road but then you'd miss other parts of the traveling portion.
If the writer had dropped the sex scene and the ridiculous letter reading scene, there would have been room but they went the direction as filmed.
He would have been fun to have on the trip but, judging by the writer, he'd be the first killed but then, with him there, you'd have to have dialog about the description of the troglodytes. Then you'd lose the ominous feeling as they reach the valley (run-on sentence, I know).
In the end, I think the writer made the correct decision.
I don't mind the route Zahler chose to go. I just remember finding the "smart Indian" (Zahn McClarnon) an interesting character in his brief sequence and was let down that he wasn't included in the long "road" section.