They are very specialised and as such have no place doing their primary role on any other fleet! yes they can be retrained to do a crewman job on helos or LM on AT and do very well at it too, however when a pilot changes fleets he is still flying a plane, when a Nav moves fleets he is still navigating, a wetman does not hunt many subs in a bloomin chinook!
People are upset that they are not getting to cross over to SH when they have spent 15 years on the kipper fleet, manning has decided that they want fresh blood primarily for their intake on courses for next few years it is tough but you can see why.