AT is fine if you are on an RAF Formed Unit. Try doing it when you are part of the so-called 'Diaspora'; moreso if you are overseas. You need to be able to hear about it (difficult if you are not on DII-F), then do the paperwork through some dismembered head in another country who requires documents to be faxed (as they haven't got access to a scanner). Then there's the issue of flight coast and T&S in transit. In my last job, I argued until I was blue in the face about this; after a while, it just fell into the 'too hard' category - especially when you are not under OPCON of one of the Services.
Not bitter - I've had my fair share over the years, but the rules need to be a little more flexible, especially if you are without PEd and TD/FD staff.