Well, unless they are carefully run and maintained you have an increased risk of pest infestations and an increased risk of fire. They should be located well away from exhibit and collection storage areas, if possible. Trash disposal should be carefully monitored. There are also sometimes problems with people trying to walk into exhibit areas directly from a restaurant, especially one that is "fast food" style, with a soda or cookie in their hands. That said, some museums have well designed, well located, and well maintained restaurants with GOOD food that is not overpriced. These can really enhance a visit for a tourist with a limited amount of time to see a museum (who does not want to have to wander the neighborhood looking for some place to eat after walking their feet off in the museum for hours!) Staff like them too!