I'm learning Arabic right now, and the Arabs seem to all hold this book in high regard. Obviously, many for merely religious reasons. However, I've found numerous people that reference it as some sort of perfected grammar. Is there any reason, linguistically, to hold this book in such high regard? Aside from its mythological value.
For many (most?) languages, there is such a thing as a standard variety among a number of dialects.
For Arabic, the standard is Standard Arabic, which is not what is actually spoken everyday in the Arabic world, which consists of several countries speaking different dialects, but what you find in the Koran. The Koran, if you will, sets the standard for correct Arabic.
If there is any perception of it being "perfect" Arabic, then it would stem from the prestige associated with the book itself.
It may also be a similar case to the language of the bible, where its words are deemed to be those of God. So to deny its status as being perfect would be to suggest God is not perfect.
This question is actually more complicated, and more interesting, than indicated in the previous posts. Classical Arabic (al-ʻarabiyyatu l-fuṣḥā), as formulated by the grammarians from Sībawayh onwards, is based in the first instance on the language of the poetry of the pagan (jāhilī) period, in essence, a poetic koine combining elements of the various old Arabic dialects. The Qurʼān is in basically the same language, but with some particular morphological and syntactic features, and quite a large amount of specifically Qurʼānic vocabulary. Muslims believe that the Qurʼān is literally the word of God, that it is inimitable and that consequently men cannot, and indeed must not try to, imitate it. This means that the language of Arabic literature of the classical period is actually quite different from that of the Qurʼān.This is rather different from (for example) English writing of the post-reformation period in so far as this consciously emulates the language and diction of the English Bible.