Another use case: if you have an ad-hoc group of developers working on a project in a decentralized fashion, they can each work on local codebases of the project without needing an up-to-date setup script or migration scripts.
I'd maybe even argue that a lot of these "goodbye MongoDB" developer blog posts are sort of unnecessary. MongoDB allows for fast deployment and the convenience of a flexible database at the cost of speed, so eventual migration to a more solid relational database could/should be part of a long-term plan when devs choose to use MongoDB. It's all about using the right tool for the job.
MongoDB also has some performance benefits over many traditional databases, and has "native" support for binary data.
I personally prefer CouchDB over MongoDB for most application that are well suited to use a "NoSQL" database, but MongoDB is sometimes decent too.
You mean in every one of your application's code. You are using a schema no matter what, but it's a missing feature in mongo.
those kinds of applications are not usually kinds of problems we have to solve.