I always figured the point of selling cosmetics, jewelry, Hermes scarves, etc. at duty free was not to offer a great deal, but a fast, convenient see-I-missed-you-honey gift for the wife / girlfriend. Many people would pay a surcharge for that convenience.

As for alcohol, whether it's good value depends purely on where you're going to drink it. Here in Washington state USA where drink taxes are high a 750ml bottle of Jack Daniels is $25. In California it's $15 to $18. Consequently duty free is worth it if you're flying to SEA but not to SFO.

When I drive up to Vancouver I always stop at the border on return to check the duty-free deals -- two 1.75L bottles of Absolut for CAD$60 recently, for example; absolutely worth it for a Washington state resident --but don't always buy.