I am a BP/W diver for 3 reasons:
1)Simplicity
2)Scalability
3)Comfort
1)The BP/W design is based on a simple harness made up of a continuous piece of webbing. It is simple in the sense that it has no useless features on it (think integrated weight pockets, quickie clips that may break, etc.) and hence extremely streamlined. Because it is extremely streamlined and not overloaded, there is less danger of entanglement.
2) It is very scalable. You can have multiple wings depending on what kind of diving you are doing. For example, you can have a Caribbean wing with less lift and a bigger size wing for cold water diving, or you can have a singles and doubles wing. Also, you can add and remove stuff to the harness itself depending on what you need on the dive.
3) Last, but not least is comfort. Since the wing is completely independent of the harness, and is located in the back of it, you don't feel a squeeze with it inflated. In a regular BCD, since it surrounds you (most models anyway), you feel squeezed when it is inflated.