From wikipedia,
For those familiar with memcached, Couchbase Server provides on-the-wire client protocol compatibility, but is designed to add disk persistence (with hierarchical storage management), data replication, live cluster reconfiguration, rebalancing and multitenancy with data partitioning.
Given this context, what are the use cases for memcached (e.g.: What advantages does lack of disk persistence confer on memcached) and what are the use cases for membase (e.g.: When memcache can work with disk persistence, what specific cases would require this feature)?
Memcached is a very simple product and serves as a basic in memory cacheing system. If you use memcached then you will also need some sort of persistent database behind. For example if you are using mysql and you want better performance you can put memcached in front of it in order to speed things up.
Couchbase is a product that contains a persistent layer under the memcached layer. This means that Couchbase is your database and you won't need something like mysql. On top of this you get a schema-less data layer, easy scalability, and data replication for always on support.
In Couchbase 2.0 which will be released very soon you will also get query support and cross data center replication.