Want to test the upgrade in your production environment beforehand? Well, make a clone a couple days early.
And once all works out, a few days later you promote the clone and destroy the old datasets. Need a rollback? Well, just start the old application instance on the old datasets, nothing touched them.
Doesn't work for all database types, especially if you have no possibility to replay new data into the rollback. But if your system allows it, it is really comfortable.
As long as you make sure to only use one filesystem, i.e. you don't place pg_xlog or some tablespaces on a different filesystem. You can get very weird corruption in such cases :)