Duty free is a ripoff for most of the items. Don't they offer it on the plane, anyway?
As others have noted, MEM provides quick connections, is pretty reliable for ontime departures, has a decent WorldClub, and two excellent sources of BBQ. NW has two other outstanding hubs in MSP and DTW, so I guess you could say MEM is NW's "worst" by some measures, but I'll take it over most other airlines' hubs. I'm sure ATL and ORD and EWR have great duty free shops but no sane person would go out of their way to connect at any of them.