IANAL, but the idea that a signature is what makes a contract legally binding is not exactly true. It is a symbol of the acceptance of the contract, but legal acceptance can take many forms - so whether you use a service to signify acceptance, or just sign it using acrobat or Adobe's site, or even just a verbal agreement... those are all valid acceptance, legally speaking.
DocuSign's use case is not the signing, but the management of those documents and signatures - tracking which documents are sent, which have been read (yes, the doc owner can get notified when you even look at a Docusign document), which have been signed, and being able to store copies of signed docs. It is mostly for the companies sending you contracts, not for you as the signer.