I saw only one example since the Devils Brigade joined Rooks, so..... I will put my 2 cents in. There was an area that I would like to have protected on the map. I noticed we had lost a port in the middle of that area and we had a CV poised to attack. A few rooks and I went at it and successfully retrieved the port. It was only after that I noticed we already had the fleet belonging to that port. The area where the cv was located was floating away from the main stream (sorry bad pun) of battle and truly would very unlikely see any battle.
From this observation alone:
1) This fleet was not needed for action and not placed in a far away corner, just outside of the main stream.
2) With the obvious fact that it's port had already been captured, while the fleet was inevitably a second line of offense to recapture it, the strategic location of the fleet worked to deny the invasion force a stronger foothole into Rook territory.
3) I saw nothing like the CV that was driving across dry land and hidden 10 sectors away from any enemy airfield in another map. Each case can be made for and against hiding Carrier fleets. In the above (#1 and #2) statements I vote for. In the more extreme case here #3, I vote hands down against.