I thought the major selling point of cloud platforms for scalable apps was that you can adapt your infrastructure under load through provisioning as opposed to adapting your platform to the hardware you've got. All the testing in the world won't expose particular imbalances that occur as a result of marketing activity, for example - so I don't see a compelling use case for cloud testing, from my perspective at least. Monitoring, yes, because it's difficult to even see problems that occur in a segment of your infrastructure on which other segments do not absolutely depend.
My question is would you rather duplicate your production environment on the cloud for load testing, or just overprovision for more spare capacity than you think you'll need, then deal with the fallout with more if required?