As a contractor one jumps between different installations of drupal for different clients. Often the architecture is baked in and needs to be maintained. My experience tells me caching solution used are, in order of frequency, 1) memcached 2) memcached and varnish. Drupal + redis as a caching solution is not prevalent even though it is free, opensource and more feature rich. I like to know why? Were Memcached and varnish available before redis? This is the answer I come up with talking to collages but seems too simple.