In most scenarios, all carriers come with associated air-groups. Additionally you get some extra USN & FAA airgroups, so if you want to offload some replenishment groups, and use those late war CVEs operationally, you have some to use. Added to that, when your carrier gets sunk, you can buy back the air wing (you might want to send any leftover fragments back to SFO to join up with them later).
So, basically, there is no need for carrier training VMF/VMSB squadrons. Having said that, putting some (small) VMSB squadrons on RN carriers can be useful.
"You may find that having is not so nearly pleasing a thing as wanting. It is not logical, but it is often true."
- Cdr Spock