I would have to assume that they're unusually well constructed? Obviously you're buying the name as well.
I'm mystified by the whole "mid century" thing anyway. It's mass produced; there's little craftsmanship, no "soul" if you will, and not much in the way of interesting manufacturing materials. And please, nobody shoot back examples such as Eames chairs built with rosewood, you know perfectly well what I mean.
It seems to be about a look, whether ironic, sincere, or both in the Warholian sense. It's hard to believe that it will still be around 10 years from now.
Still, any interest in the past is better than no interest in the past.