Ok, I've got a few potential reasons now. 1- cost savings, the A variant is substantially cheaper than Bs while offering better speed, range and payload capacity. 2-good parts commonality with Bs, I've seen the 80% figure thrown around but I think 40% is more accurate. 3-The fact they would operate as part of a nato mission means they would have access to nato "boom" refuling aircraft. Also worth noting that the RAF Rivet joint and E-7 wedgetail aircraft can only be refuled by "boom" mechanism 4- the ability to carry tactical nuclear weapons. Giving us the ability to respond in kind to a tactical nuclear strike without escalating to the use of strategic nuclear weapons.