How do you feel about co-workers who attend work potlucks empty handed?

It’s happened more than once at the office: There is an office potluck. It has been announced for several weeks. Everyone at the office knows about it. Then comes the big day. There is bound to be several co-workers who straggle in… with nothing in hand except an empty plate and cup for healthy servings of “free food.” I used to be more tolerant of these co-workers, but it gets old. I spend money and time buying and making food for office potlucks, why can’t they (if they want to participate)? How do YOU feel about co-workers (or other people) who participate in potlucks, but don’t bring anything to share?