Agree re the Alcantara being a purely personal preference. It came w/ my P85 at the time, but I would have paid extra for it if necessary. Saying that it is not cost-effective if you have the Pano roof (because there's less square footage of coverage there) makes no sense to me.
From the driver's seat, you see about the same amount of headliner material either w/ or w/o the Pano roof. IMO, the Alcantara looks and feels much richer than the somewhat coarse standard textile headliner weave, and many of my passengers have commented on the nice texture of the Alcantara.
So if you're trying to save money by omitting the Pano roof, then I agree that you should also skip the Alcantara. But If you get the Pano roof (which I really like as it brightens up the interior all the time, and is really cool to use at least a few days a year), then I would also get the Alcantara, as it makes the interior feel that much richer all of the time.