Surely when the franchises were created, they should have had a fair split of staff anyway??
Operational convenience and route knowledge have a lot to do with these type of arrangements between firms. On the Cotswold Line, for almost all of the time that Thames Trains ran the route, many of the Turbo services were driven by Central Trains drivers from Worcester, who had route and traction knowledge from
BR▸ days, while the guards were Thames staff. I believe the arrangement only came to an end because Central was beefing up Worcester-Birmingham services and could no longer provide enough drivers.
In the case of services between South Wales and England, a lot of
ATW▸ staff have route knowledge dating from BR and Wales & West, so why not use it if they're available? Let's face it,
FGW▸ doesn't exactly have staff coming out of its ears. Remember Christmas and New Year! I assume some of the new FGW recruits will be replacing bought-in ATW staff eventually.