A lazy, pull-only, continuous replication server from CouchDB to MongoDB.
Because I'm a lazy programmer.
Because I wanted the ability to write a geospatial-aware API server with a roles-based ACL, but I didn't want to write either of those features themselves. CouchDB has a sufficient ACL implementation, but it doesn't support geospatial queries. MongoDB supports geospatial queries, but it's ACL implementation is practically non-existent. Since MongoDB is a master-slave setup anyway, the only conceptual change here is that the writable master is CouchDB.