Pheasant,
I agree with you that 'joint' doesn't work all the time. There has been intense rivalry between the 3 services, especially regarding ownership of rotary, for many years, ney decades - I think that even I can work out which side of that particular fence you sit on. But, I could just as easily pose the question 'why have 2 services (Army and Navy) operate rotary when it could be done by one?'. I am, of course playing devils advocate there. I think the crux of the matter for the current regime will be cost - will it cost more to keep the Merlin Mk3 under light blue or transfer it to the army, where the output lies, or transfer it to the FAA? The cheapest option is likely to win the day (again). 'Joint' is often painful, but it does work in some areas!
The final outcome will cause pain somewhere.