The SRA Route Utilisation Strategy for the London Brighton corridor has identified that the Gatwick Express paths are not particularly efficient when you consider the number of pax shifted. Trains are busy during the morning but come the afternoons they become quiet, additionally platfrom occupancy at Gatwick is a problem. These paths could better serve the tax payer by extending southwards.
The current Gatwick Express service obviously uses dedicated rolling stock that was designed for airport passengers and all that goes with them, specifcally luggage.
I suppose BAA are worried that the ambience on any revised service pattern would be degraded, more crowding and the possibility of rolling stock substitutions where there is less luggage capacity.
BTW, a half hourly stopping service on Heathrow Express commences in Spring 2005 called Heathrow Connect and will serve Ealing Broadway, West Ealing, Hanwell, Southall & Hayes before heading for Heathrow.