Arabic speakers in Dubai are in the minority
But its the official language so surely the one used in formal/business setting, hopefully along with English!! I guess the others are Urdu and Hindi are used by a lot of the people who are living there at the moment.
It is hardly a neccesity to speak Arabic in Dubai (English will most often be used, most people in Dubai speak excellent English ...or atleast the ones you will be dealing ith in business), but it will look very good if you can speak Arabic as it shows you have made an effort. It will be greatly appreciated.