The increase in A-A capability that the GR fleet got as a result of equipping with ASRAAM was a secondary benefit, most definitely not the driver behind the change. The reason, surely, for the change was to rationalize 1 IR missile for all RAF FJs. Thereby saving money in support costs. ASRAAM and no AI radar doesn't really increase self defense capability a whole lot as far as I am concerned. Especially if you have any sort of ROE/ID criteria above 'weapons free' and that assumes you can work out who is friendly.