Fusha: it really is your linguistic passport
After reading about wildly differing Arabic dialects, I expected nothing but linguistic disaster when I came to Egypt. I hoped I would be understood, but I thought مصري was going to be as comprehensible to my ears as French. However, when I went and started talking, I was surprised; conversations were not an exercise in hand gestures and exaggerated syllables. Changing the ج to Giim and the ق to ء takes mental processing, but with a little lag, a مصري sentence can be mentally translated into Fusha, and a Fusha reply has been perfectly adequate for understanding.
Coming to Egypt proves utility of Fusha for the travelling Arabic speaker. Lapses into نجدي Arabic have been universally unsuccessful. Anything in my pseudo-gulf colloquial, like أبغى or وش was either a subject of quaint amusement or blank stares. Fusha also means that the ‘expat Arabic’ dialect is also easily understandable. Regardless of origin, it seems that almost all foreigners learn a Fusha and only switch out the most common 30 عامية words as they live in the region. This applies to Chinese in Riyadh, Germans in Damascus, or Americans in Cairo. The only other real change is swapping a few letter sounds- and a pseudo-local accent is born. Ultra-common words like زين، عيز، and فراخ can be inferred from the surrounding Fusha sentence, and then ‘expat-Arabic’ is easy.

If only there were an actual visa that could gain access to the entire Arab world!
Finally, the most important part about using Fusha outside of an adopted عامية region is that it’s an acceptable, if stuffy, dialect to native speakers. For an English analogue, using نجدي outside of Saudi Arabia would be like using a thick Indian accent in America. It could be understood, but it conjures many stereotypes and can inhibit transmitting any form of subtlety. Fusha, on the other hand, is talking like the Economist about a cheeseburger- certainly out of place, but mostly understood and still possibly respected.
While I still may resent many hours spent learning the dual and ‘arab, my trip to Cairo reminds that Fusha still serves as the only real linguistic passport in the Arab world.