I absolutely think the horse should 'graduate' to a double. It is a big pet peeve of mine seeing horse's put into doubles before they're ready, because the rider is having trouble with them in a snaffle so uses the double for a control mechanism rather than for refinement of already established training.
I am a firm believer in that a horse should be able to perform all Grand Prix movements in a snaffle, as well as in a double.
As Maura said, if you look at a good Grand Prix combination, you will see that most of the contact is on the snaffle rein, there is generally a loop in the curb rein that is only employed occasionally when needed. This is what you want to see, not riders that swing off the curb *cough* Ulla *cough*