[ablankst]

From [1] "in the case of blocking workloads, it’s extremely difficult to determine the number of threads that optimizes overall throughput because it’s hard to determine when a request will be completed." So the argument they make is that responding to the number of blocked threads directly could lead to an over-correction, where you add a lot of threads to the thread pool when threads are about to unblock. This reduces throughput because you now suffer context switches and poor cache locality.

[1] https://msdn.microsoft.com/en-us/magazine/ff960958.aspx

[obstinate]

This is only a problem when you're not scheduling cooperatively. If you are scheduling cooperatively, then you don't ever have to experience unnecessary context switches.