I haven't heard this about keeping sewer pipes clean, but I'm not sure I believe that either. They like to be clean, and wash themselves frequently, like cats. I don't think they're any worse about carrying diseases than the cute animals people like to see around. The worst rat diseases I'm aware of can't be spread to humans, but only to other rats. I know they can damage buildings and food supplies, but I think their negative impact in cities is exaggerated simply because many people find their appearance gross or creepy. Their behavior isn't that different from that of wild animals that people enjoy having around.