I think one of the main reasons to get a Negril driver to pick you up and drop you off is that the drivers outside the airport are Mobay-based drivers and they usually want to charge a little more because of the business they miss out on when they take you to Negril and, of course, they have to come back empty.
As someone else said, the Negril drivers are often offering a free cell phone for you to use and now you will know a driver to call when you want to go to the beach or the cliffs or shopping etc.
Another reason I would like to add is if this is your first trip, you now know someone who lives somewhere around Negril and can fill you in on the "runnings" (especially if you want to partake of the local culture).
Peace and Guidance