At the risk of pushing the boundaries, the vomet comet in an essential part of most operations throughout the world. You neaderthals need to look past your cold war beer bellies and realise that the kipper fleet is evolving all the time. No longer is it sub hunting and home for another curry. If this were the case, then why is it we've been packing our buckets and spades, for the past few years.
To get back to the real conversation, the streaming process takes into account all the exams taken, the dom flights, your character and other factors, before deciding which discipline you're most suitable. Then, of course, we are all bound by the RAF and our posting peeps will then allocate the requisite numbers to each specialisation. It may pan out that there arren't any fixed/rotary slots when it's time for streaming.
End rant