Disney definitely needs a grocery for DVC guests at DTD. Everyone can get to that location by bus or car from their resort. The grocery stocked at each resort is different, SSR carries Saratoga Salsa, others don't. OKW has the most extensive list of items. Others barely cover the essentials. When at VWL, hike over to Fort Wilderness, and you can find fun grocery items not at the WL store.
When at BWV last week, we had no dishwasher powder upon check in. I decided to just buy some at the Green Door. They had none in stock, and I was told they no longer carry it. So I went to the front desk to have some sent up to us. In a similar vein, the store off the lobby at BWV carries items not to be found at the Green Door, such as hand lotion, newspapers, etc.
Personally, I think all DVC resort stores should be exactly like the OKW store. It has the best selection, and carries all items in one place.