When Anonymous attacked Visa and Mastercard via DDoS (in retaliation to them cutting off Wikileaks donations), Anonymous did actually succeed in stopping the online verification systems for both companies (SecureCode, or something, and Verified by Visa). In that case, the DDoS attacks did more than just take the site down; they financially hurt their target, which was probably the aim to begin with.
I'm not justifying the attacks and I agree that they are the wrong way to go about this business, but it would be naive to suggest that the DDoS attacks are a minor inconvenience.