valuable for several reasons. First of all, they serve as a checklist of all of the parts of the task. For example, programmers almost always view coding as the largest part of the task. When coding is done, they believe that they are "97% done." The checklist reminds them that there are other, equally important, tasks that must be completed.
Inch stones are also invaluable in overall project tracking. Take, for example, a programmer who has burned 50% of the budget, but has completed only 30% of the inch stones. You can then multiply the remaining budget by 5/3 to get an expected cost at completion for that task. You can then use this to adjust your completion date for the scheduled task.
Inch stones are binary - they are either done or not done. Also, the completion criteria for inch stones should be extremely simple and easily verified. For example, the completion criteria for unit testing a module might be that the unit test plan has been executed and the unit test record checked into the revision control system. When receiving status on inch stones, you should repeatedly verify that these criteria have been met until it becomes reflex for your people to complete them.
A disagreement over the meaning of done is always a recipe for disaster as it allows you to proceed as though everything is fine, when in fact it is not. You often don't learn of your problem until it is too late to do anything about it.