That doesn't work if you need to commit for one reason or an other. And while in some cases you might be able to substitute subtransactions via some sort of test mode, it's not always an option depending what you're doing / testing exactly.
Nested transactions handle commits. At the beginning of a test, a new transaction is begun, and it is rolled back at the end of the test. As others have pointed out in this discussion, it's pretty standard with Ruby on Rails apps, especially those which use rspec for testing.
