This brings back somewhat painful memories of a large project post-mortem where the CEO continually asked "Well, was there a project plan?" (and didn't ask anything else). In this specific case, there was a decent project plan, but so many other things (that the CEO chose not to acknowledge) went wrong that the project plan was not able to salvage the project.
Jim Vaughan, in the IT Project Management Blog, also asks (from a much more informed perspective of course) why so many projects with a project plan are failing? His explanation is that too many project managers will open up their slick & sophisticated PM tools and start entering tasks, since the task list is the first thing they will see. This process is a more bottom-up approach (or inside-out, as he says).
His solution to this problem:
...is not the tool but rather the methodology that the PM chooses to use for the development the project plan. The best way that I have found to do this is to gather the technical experts together in a large room with a blank wall. Together, under the leadership of the project manager, using sticky notes, create the WBS from the top down on the blank wall. From there the data can be entered into the tool of choice. This methodology will produce a much more effective plan.
Agreed, that working together with the experts will lead to much more accurate and reliable project plan, but let's not discount the advantage that a powerful project management and collaboration system can give the project leader in setting up and maintaining the plan throughout the project. With web based project management software where all team members can log in and collaborate on each task in the project, the project plan will no longer be something that one person maintains in their own little bubble (like I used to, back in my MS Project days). With a system that allows team members to estimate their tasks in ranges and add new tasks once issues crop up or new challenges arise, the project plan will continuously be relevant.
So, take Jim Vaughan's good advice in building your next project plan, but make sure when you do that you are using a project management tool (albeit somewhat slick) that will allow you to keep your experts engaged and connected throughout the project.