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I think it would be helpful in theory, but in the US I don't think it is likely that companies will voluntarily adopt a standard that isn't drafted by their own lawyers or perhaps their own trade association. It would be like asking AT&T to adopt a customer friendly service contract - it isn't going to happen, because they can stack the deck in their favor, and you can't do a darn thing about it. I don't view most of the major cloud service providers as negatively as I view AT&T, but unless everybody is on board at once, I can't see one of them agreeing to anything that might cost them money through increased obligations to customer service.
As to the specifics of the ENISA guidelines, I think they are actually quite good. While I may have doubts as to the adoption of them or something similar as an industry standard for contracts, I think they could be very helpful guidence for companies entering "The Cloud" for the first time. The ENISA guide helps ensure that, if they are followed, the right questions are at least being considered, and the company entering into the contract will understand what they are getting and the responsibilities of the could provider under the contract.