When I lived in the U.S., I never really understood the point of duty-free. But now that I am overseas, I see the value -- for example, alcohol at IST inbound duty-free is much less expensive than in Istanbul. Same with cigarettes or cigars when traveling to Europe.

It depends on the product and where you are going, but there are certainly times when it's a good value. Keep in mind also that there are times when it's the most convenient way to purchase a product, e.g., alcohol going into DXB. You also see this with people carting all sorts of seemingly pedestrian products (like huge cans of Tang powder, which always amuse me at DOH) when traveling to third-world countries where consumer goods are not so abundant.