why on earth would you want to have mongo as main database to emulate relations?
- No manual database/user creation needed
- No manual table creation needed
- No schema migrations needed
- No nested relations needed, because typical web app data structures fit well as dictionaries/lists
- No extra caching layer needed because of previous point
So basically, your app is simpler, and when you give it to a co-developer who has MongoDB installed, he can just start using it. And when you develop further, you don't have to mess with the schema migrations either with co-workers or when deploying to the server.