it is mostly an Officer issue. The promotions are very competitive--to the point of being nearly arbitrary--everyone looks the same. So, what happens is that people get de-selected instead of selected--oh, this guy looks different, we can easily rationalise seperating him --
Enlisted people have an exam that can seperate them--though in the higher e ranks it becomes similiar.
The Army is a lot more flexible and is a pretty good model where you can move in and out of specialties. Though I am not an expert on their promotion system. Conversely, their SOF has to serve outside SOF, which can be seen as a destractor--or at least it is from the Navy perspective.
If you can answer your wondering question, than you should tell me, cause I haven't been able to make the case to the powers--
It is dangerous for those with security clearance to comment on the existance of some units--and my SC is stil active :)