Makes a whole lot of sense if it is the budget end, i.e. holiday and tourist market they are chasing. It is only a 30 dollar train or bus ride from Brisbane to either Sunshine or Gold Coast. Astute business travellers may even embrace the idea eventually.
By the time you figure in gas and parking or at least a cab ride from most of the 'burbs around Brisbane to BNE Domestic you may as well go a bit further out to catch a flight.
The way traffic congests around BNE now, it would not take up much more time, either.
Melbourne has Avalon which works OK for Jetstar, but unfortunately none of the other big cities have anything an equivalent time away. Otherwise we would see a lot more of this concept. Works a treat in Europe and the USA. Good on Gocat; if they stay cheap they will get my dollar.