[edwinnathaniel]

If your code has a hard-static dependencies to web-service call, you can't test it easily (and repeatedly). Especially when the actual service is down.

If your code has a hard-static dependencies to DB call, you now have excuses not to write test against it because "it's hard to prepare the environment to do such things."

Not all projects are as small as a Todo-List written using Rails.

Medium-to-Large size projects with large DB schemas exist in which it will take hours to run your Rails tests: "testing-my-active-record-models-as-unit-tests"