@LSCatilina
As an Arabic speaker, I haven't actually experienced any of the fracturing or incomprehensibility of Arabic that people in this thread are talking about. While there are differences in dialect and some are more understandable than others, in the end it's pretty easy to understand other dialects with some effort and context clues to make up for strange sounding words. Usually the people who have issues with dialects are often stilling learning the language or aren't native Arabic speakers so dialects may be daunting for them without the context that is given to you by living in the Arab world.
Well, most Moroccans I know (Moroccan Darija native speakers) speak to be unable to understand either Fusha or other distant vernaculars properly, though they manage quite well with other Maghribi varieties. Similarly for other Arab speakers I know (and and I know quite a few).
But it is true that, as a speech community, Arabic has not fully fractured (except Maltese as noted), as in the formal language is still unitary and used in the appropriate contexts (by educated speakers at least) even though the linguistic system that most if not all Arabs acquire natively is the vernacular one. The point is that talking about Arabic as either unitary, fractured, or even diglossic, is incomplete: Arabic is unitary as a
range of vernacular and increasingly formal varieties boperating along a mostly contextual continuum; they, however, are by themselves different enough to be describable as distinct languages in terms of purely structural divergence in stuff like grammar. The sociolinguistic and educational systems, maintain a collective level of general mutual intelligibility among the educated portion of the population, and also tend to
disseminate both the standard formal variety and the educated register of the most prestigious spoken varieties (usually the ones of the capital cities).
A linguist who looks at Qur'anic Arabic and modern spoken Cairene in terms of their grammar, phonology and almost any other structural feature of note would then be correct in saying they are distinct languages (though of course related) under the specific perspective of the discipline. Without specific training, acquiring each does not allow proficiency in the other; conversely, an anthropologist or a historian who look at how they are actually used by the people would quickly realize how in concrete they function as parts of a larger way of defining the "Arabic language" as the whole of the system by which Arabs do indeed perform different kind of linguistic acts approprately to a given context.