SSL Certificate - Internal CA vs "well known CA"

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Dear List,
Just wanted to understand why using a "well known 'trusted' CA" (e.g. verisign) is more secure than using an Internal 
CA to manage Certificates

e.g. if a company wants to publish a non-financial site (as opposed to, say, Internet Banking) would not an Internal CA 
be as Secure as an external one?

What is the real (security) benefit of using (expensive) external (e.g. Verisign) Certs?

Thanks you for your comments