We have just had our first contract tester and I have to say that it has been a very positive experience.
Our QA environment highlighted the need for a change in design.
We work with Web content management systems so we can't simply add tables into the application database, there has to be a 2nd, and sometimes 3rd database supporting bespoke functionality.
The problem comes when a query in the main app database references an external database.
This means that we have to change all the queries that reference database.dbo.table. Makes setting up a QA environment a bit difficult.