The workbook only needs to be filled out to the extent that it is useful for the Scout.
Lack of a final plan, or one that is not completed to the satisfaction of anyone other than the project beneficiary, must not be used as a reason for unit leaders to refuse to sign a project report. To do otherwise is a misinterpretation of the Guide to Advancement, and essentially adds an advancement requirement. If the approved project was completed to the satisfaction of the project beneficiary, then a certain amount of planning, development, and leadership must have occurred—even if it is not documented by a final plan.
While I think I understand what the National Advancement team is trying to do, I think it may cause some Scouts/Parents/Leaders to read it as the Workbook is completely optional. To me, this goes directly against the wording in the Eagle rank requirements that says (emphasis added):
While a Life Scout, plan, develop, and give leadership to others in a service project helpful to any religious institution, any school, or your community. (The project must benefit an organization other than Boy Scouting.) A project proposal must be approved by the organization benefiting from the effort, your unit leader and unit committee, and the council or district before you start. You must use the Eagle Scout Service Project Workbook, BSA publication No. 512-927, in meeting this requirement. (To learn more about the Eagle Scout service project, see the Guide to Advancement, topics 184.108.40.206 through 220.127.116.11.)
I understand they want to avoid making the paperwork become overly important, but I am afraid of the precedent they may be setting with this article. Maybe I need to stew a little more on it. BSA has used the word "must", which seems pretty explicit, but now seems ambiguous given the recent article. I can just see Scouts turning in the Contacts page now as their sole use of the Eagle Project workbook or something equally sparse like putting in their name. If you say "must", then say it doesn't have to be used, it will lead to confusion.