With less emphasis on basic stick and rudder skills these days, the focus is now more on 'spare capacity' than in the days of yore. Sure, basic military pilot aptitude is assessed at OASC, but it is only with progress through the system that an individual's spare capacity can be fairly assessed.
As far as I'm aware, the crew operating concept for the FSTA hasn't yet been finally established. The basic A330 normally spends its life in protected airspace under positive radar control on a flight plan which rarely includes significant changes en-route. The odd 'direct' or minor re-route for sure, but rarely complete mission changes. The routine Airbus workload has been optimised for PF/PNF duties in such controlled circumstances and most pilots can cope with it pretty easily.
But if you now add additional tasks necessary for AAR, particularly if, by virtue of non-optimal flight deck design, those include tasks which would be better allocated to the mission specialist, the need for additional mental capacity and flexibility for the safe management of large formations in unprotected airspace becomes significant.
In particular, it's no longer a question of simply doing what you were told pre-flight. As a wise old Victor person once said "The only thing certain in the AAR game is the time you came to work".
And yes, I have 'written a book'. More than one in fact. But nothing which you could buy in a shop - they were military AAR manuals.
The RAF has a good reputation for the standard of its AAR force. The aim being to offer the most flexible and efficient service possible to receivers, but without fuss or drama. I sincerely hope that such standards will not be eroded in the forthcoming PFI era.