When you understand this [queueing theory], you see that idle time in development is not 'waste', but rather a tool that prevents your resources from being blocked 100 percent of the time. This gives you insight as to why a company like 3M builds 15 percent excess capacity into their development process. Do you really think they do it because they like 'waste'? They say it helps make them more 'innovative'.
Reinertsen is the author of the excellent Managing the Design Factory: The Product Developer's Toolkit, which I read a couple years ago. Maybe it is time to reread, now that I've had more experience with TOC.