Foil isn't screwing around with food, despite my somewhat flippant remark earlier. :shock:
What BBQ cooks are trying to do isn't all that different from fine dining. We are trying to bring layers of flavor and complexity to the dish. A one note dish (porky, smokey, etc.) won't win, even if your friends and neighbors think it rocks in the backyard. A balance of hot, smokey, sweet, and savory will do better. Do you build a sweet profile and offset with acid in vinegar? Do you have front heat, back heat, or both? Will your dish be hot enough for those that like heat, and not be overpowering to those that do not? Do you honor traditional BBQ flavors, or try to introduce something different to set you apart?
Good cooks will consider these questions, and foil is a useful tool to introduce additional layers of flavors into the dish. It's hard to add savory elements like Parkay, acidic elements like vinegar, and sweet sugars (without caramelizing them) late in the cook without foil. Foil can also give you a wider window in which to make the product tender without drying it out. Can people win without foil? Of course. It is, however a useful kitchen tool for the BBQ cook.