I think the general thinking behind the more expensive fully synthetics is that they will withstand higher temps, better for trackdays or constant hard driving. I think in a road car, you will be hard pushed to generate the kind of oil temps a track day will generate, so a good semi is more than enough for road use.

Secondly, a fully synthetic will stay in grade longer. Ie a good 5w40 fully synthetic will stay as a 5w40 for extended service intervals. I semi-synthetic 10w40 will not stay in grade as long. However, the cost difference would mean you can change a semi-synthetic 2-3 times for the price of some of the fully synthetics.