Apparently, efficiency - separates ship and flight ops completely. I can think of other pro's and con's, but the only reason I have heard is that of efficiency by separating flight ops and bridge.
USN is sticking with the single Island deal though; just smaller and further back on the newer ships. The USN reckons this provides more room/efficiency for turning (rearm and refuel) a/c, to increase sortie generation rates. This is probably more of a consideration on a ship that will carry 80-90 a/c, as opposed to the Brits who will probably embark no more than 36; same is likely for the Russians and Chinese (who are looking to embark up to 40 a/c on their new ships).
D