Namely a context like travis-ci where you get a new clean environment each time.

Personally, I've develop few libraries/web UIs that relies on external software like a DNS server or an ldap directory.

And for those, I've a quick and dirty shell script that deploys bind/OpenLDAP.

It's far easier, faster and more accurate than mocking.

For example, what comes to my mind is the testing I do for all the SSL/TLS modes I support (SSL, StartTLS, certificate check disable or not, etc) in my ldap web application.

Travis also has available services in their build envs (stuff like redis, mongodb, mysql, cassandra...).