If I remember correctly, the reason I specified synthetic workloads is that I was afraid that if I specified just workloads in general, the reader would visualize real workloads and consequently would object to the whole idea because of the loss of privacy. There is also the issue that when someone is doing real work, he is often unwilling or less willing to switch from doing that work to answering a question about the work (e.g., what are you trying to achieve right now?) and I thought asking questions of the user would be useful. Of course, there are no universal reasons that real workloads cannot yield satisfactory data.