The gut in mains vs. gut in crosses are for different purposes.
The gut in the crosses is to take a poly stringbed and to make it more comfortable and slightly more powerful. Since the mains are the primary factor, the gut crosses kind of change the flavor of the poly string job.
The gut mains/poly crosses is different than a full gut string job. Mains ride across the poly crosses like on rails instead of the whole stringbed flexing on a full gut job. The springiness of the gut gives power (though less than full gut) but the stretching and snapping of the gut across the poly can give great amounts of spin.