Can someone explain what kind use-cases there are for a database where the client can read and write directly against the database? CouchDB could do this (via

Can someone explain what kind use-cases there are for a database where the client can read and write directly against the database? CouchDB could do this (via CouchApp), but I bet not many people do this because it's bad from security and scaling standpoint and I think there aren't that many types of apps that can be created without a proper back-end.