Imagine a project manager who pays a very small amount for a very advanced cost and schedule control system, and the system reliably produces accurate information in a timely manner. If that project manager ignores that output, and instead acts out of impulse, then the value of the project management information system is zero.
And, to be blunt, no amount of eat-your-peas hectoring from our side of the debate will lead such a project manager to have an epiphany and turn away from his or her nefarious ways. Conversely, a project manager who pays a great deal for a system that barely reports projected costs at completion and forecasts completion dates, but uses that information to avoid a massive overrun or delay, has an extremely valuable system.
Not to abrogate the debate, but, in my opinion, the value of project management is like the macro-economists say: It's what somebody's willing to pay for it.