DBM and similar databases are simple on-disk key-value stores. The use case is a lot like a SQLite, where you just need to put some data on disk and retrieve it efficiently later.
Can CouchDB be used in that fashion? Seems a lot more complicated, more like MongoDB than anything.
Definitely more complicated than a key value store, or sqlite, more like a distributed document database with (optional) map/reduce views (indeces) and more recently mongo-like indeces.
Best use case though, for me, is key/value durable data storage that you can sync (two ways, no need for master).
As for performance, its pretty fast, but not redis fast as data is written on disk using btree indeces. Retrieving multiple values (documents) with similar looking keys (ids) can be quite fast.
Can CouchDB be used in that fashion? Seems a lot more complicated, more like MongoDB than anything.