Is memcached worth running on only one server?
Yes, Memcached is great to run on single servers - but it especially excels when sharing cache results between clusters, speeding up each host as well as the cluster as a whole by eliminating much of the processing and look-ups cluster-wide.
Running Memcached with W3 Total Cache works brilliantly, it caches tons of stuff automatically as well as giving you the ability to directly store the WP database cache and object cache there too.
However with WordPress and single server installs I would maybe suggest you use a php accelerator instead, something like APC.
APC is primarily an opcode cache designed to speed up php execution by pre-compiling it and serving up the bytecode from a shared memory cache. It also gives you the ability to store keyed data (the database cache and object cache using W3 Total Cache) just the same as memcached.