I do think that a lot of people have gotten the hostess gift concept and the potluck concept mixed up.
A hostess gift is a small gift taken along to express thanks for the hospitality, and is something that is *not* expected to be used at the meal. It can be a bottle of wine, chocolates, flowers - I might take a jar of home-made preserves, or something like that.
A potluck is when guests bring food items that are intended to be eaten as part of the meal. A hostess gift is not required for potlucks, because it's a form of shared hospitality.
It also doesn't make sense from a basic logic perspective. You're holding a dinner party for six people. Each of the six brings a different dish, with the intention of eating it at that meal. You've got close to a full meal right there, excluding the full meal the host has prepared. It just doesn't make sense.