Security is not a Luxury
Vulnerability testing is not a luxury. On the Internet you are no longer in control of how your application is used. It's foolhardy to expose your systems to the threats of the Internet without checking to ensure all the potential holes are closed.
Proactive vulnerability testing reduces the risk of what might happen if the bad guys decide to check your web application for security holes before you do. If you end up a victim, you can consider yourself fortunate to merely find yourself hosting malware or stolen music and video files. Even then, imagine how you'd feel if a customer contacts you and says their computer got a virus simply from their browsing your website.
Avoid the Expense of a Break In
Vulnerability testing allows you to identify and resolve the weak points in your web application code for a fraction of the cost of a single successful break in.
Learn about Your Security the Easy Way
Vulnerability Testing allows you to test your applications with no threat or malicious intent towards the systems or the data they contain.
Keep your Data Secure
Confidence in the security of your web application also means confidence in the integrity of your data. You know it's not being stolen or sold on, you know it's not being altered or manipulated, and you know it's not being deliberately corrupted.
Vulnerability testing, when bought in as a consultancy service, doesn't require additional headcount or office space, and is often inexpensive enough to fit within a software team's discretionary spending budget. It reduces the risk to your organisation of the embarrassment and potentially astronomical damages associated with a hacker break in.