Now then, back to the Gatwick Express ( which is what this thread started as ), jabird, I think there may be a misunderstanding - BAA DO NOT own or operate the Gatwick Express and therefore cannot charge heavy monopoly fares for this service - it's run by National Express !
I think the point made about travelling from the airport is very valid. The thought of boarding a packed commuter train coming up from Brighton in the peak morning period, which co-incides with Gatwicks busiest arrival time, just doesn't appeal - it will be crap, crap for the commuter and crap for the airport passenger.
I have used the Gatwick Express on a numerous occasions and although it rarely does the trip in 30 mins ( 35 is average ), it is actually a pretty good experience.
It is possible that BAA would need to build more car parks if the service was withdrawn and that would go against the development plans for the airport which contained a clause around more / better public transport access.