From a product page for Carbon Black, a similar material to the stuff Apple uses, these look like the advantages over a phase change thermal pad:

- Fortified with glass beads to maintain uniform bond line over the life of the device

- Eliminates die/component hotspots caused by incomplete heatsink contact seen with use of other thermal compounds

Additionally, a thermal pad requires a presssure mount with a narrow gap, while Carbon Black is designed for non-pressure-mounted components.