I spend what seems like a lot of time working with schedules and scheduling tools. But in practice the visible product of that work offers a pretty poor return on the effort that went into it.
To illustrate what I mean let me give the example of a large project I'm familiar with. This project involved roughly 2000 people working full-time for several years. Project schedules were developed and tracked at a high level of detail with tens of thousands of activities and a requirement that every person posted the details of their daily work. The end result was a schedule model and some indicators that showed the project was going to be late.
If you take a look at the effort that went into creating and maintaining that schedule and the resulting indicators you would find it was a couple dozen person-years. Now perhaps on a large project that is not too large a price to pay, but in absolute terms it seems immense.
However, on this same project, in some back cubicle someone put together an informal pool and kept a list of people's guesses about when the project would complete. None of these guesses were as optimistic as the results that the schedule produced. All of these guesses were closer (by several months) to what actually happened than the schedule projections were for most of the project. The cost of these guesses was a few person-minutes a piece. Based on this I did a similar exercise on another project. At the beginning I asked the people working on the project when it would finish. I got a range of several months on a project which was to last about a year and a half. It turned out that the average guess was within a week of what actually happened. Once again it took about an hour to actually do this.
My purpose in this is not to say that schedules are useless. They certainly are not. But it is clear to me that their value as a predictive tool is vastly inferior to the informed judgement of the project participants.
Now one can say, "well, if they put what they really think into the project schedule then it will be correct", and in a perfect sort of world I'd be forced to agree. But the world is not perfect and a schedule is a necessary abstraction of what is planned so there is some detail that is lost and some assumptions which can not be embodied. One can not create the Mona Lisa with a box full of dry erase markers. However, the human mind can imagine it with minimal effort. Why not use that same power to predict when your project is going to complete?