all good points, the high speed trains offer only a few trains a very limited service.
Perhaps we should ask the 300,000 who used EUJET in its very short life, why they used it.
I lived at Tonbridge at the time and found MSE better that Gatwick, but to fair it all depends what time your flights are.
Going to the Major airports by road you need to build a few hours for delays and Heathrow I found to be a nightmare round the M25.
For longhaul better to go the day before and stay in hotel for a morning flight.
Using trains from the south east you have to cross london to get to all the Airports, except Gatwick which is very well served by train from London, but the cross county services are poor with no late trains.
I did use Southampton a few times by train from Tonbridge changing at waterloo which I found very easy.
The last factor is price, if the price is right the punters will come.