"Isn't 'Warp' within DigiTech's GNX 3 just pan or a crossfade?"

No. Warping extracts the characteristics of each amp model and combines the two into a completely new sound, whereas panning or crossfading simply increases the level of one while decreasing the level of the other, while not actually combining their actual characteristics. The warp control changes the ratio of the characteristics between the two amps (for example: 25% Fender, 75% Marshall). Both methods of combining sounds are useful, but do sound quite different from one another. The combining abilities of the GNX3 are made possible by modeling technology that hasn't been readily available until the last few years. Once you've created an overall ratio between the two amps that you like you can actually save that as an individual amp "Hypermodel." Expect to see more of this type of thing as modeling amps and processors become more ubiquitous.