As a customer, security is important when you are testing your internal, firewalled, infrastructure. This is one of the main reasons Sauce Labs (where I work) gives every test its own VM that's destroyed afterwards. Using a shared or reused VM opens the door to malicious code running in the background.
There's an article devoted to this topic here:
Security Through Purity — http://sauceio.com/index.php/2011/09/security-through-purity...