Disadvantages of using thrust vectoring only in an aircraft with variable rotation nacelles such as the X-19
It is my belief is it would allow the use of highly complex airfoil geometries that would increase the efficiency of the aircraft.


The best airfoil is one that adaptively changes shape to meet current requirements (read: flaps or flaperons that run the entire length of the wing).


  What would be the advantages of using thrust vectoring only for control of roll, pitch and yaw?


That's how the V-22 works. During horizontal short field takeoffs, the engines are angled forward and upward and the flaps are fully deflected to generate maximum lift. Pitch and roll control are provided by thrust vectoring and differential thrust (the V-22 has conventional swashplates). Below is an image of a V-22 during takeoff. (source)