To add to Isa's remarks there, I would guess that the only Arabs who would have no basic grasp of the sound and feel of Modern Standard would be Arabs brought up in the diaspora, of the 2nd generation let's say, who do manage to speak their vernacular at a manageable leve. Anecdotal evidence seems to suggest that there are some of them who would draw a blank upon hearing the simplest of sentences in Modern Standard.
As for Moroccans, `Isa, some of them seem to function on three registers, in fact. In addition to M.S.A. (I wouldn't know the level of competence with which they speak in or recite Fus'ha, but I think a unique accent is quite detectable) and their unintelligible vernacular, which is so much its own kind that I cannot distinguish between when they are using it and when they are speaking in Berber, they 'modulate' and switch to an intelligible 'middle' vernacular closer to those of us from Northeastern Africa and the Levant when conversing with us. I did not communicate with the Moroccans I know in M.S.A., and neither did they with me.