Final answer:
Spivey didn't see value in Dunbar's journal due to a lack of historical significance and a bias towards written material. The writings may have been used to promote the 'black legend' against Spain and English exploration. The omission of some aspects doesn't make the scroll untrustworthy as a historical document.
Step-by-step explanation:
Spivey didn't see value in Dunbar's journal because of a lack of historical significance and a bias towards written material over oral tradition. The writings may not have aligned with the prevailing narrative of the time and may have been used to promote the 'black legend' against Spain and subsequent English exploration and colonization. Additionally, the omission of certain aspects of Song society from the scroll does not necessarily render it untrustworthy as a historical document, as historians consider various factors when evaluating the reliability of ancient texts.