I think you might be talking about something slightly different -- the question at hand isn't whether or not a GDD must follow a rigid format, but whether you should stick to it rigidly once it has been written. A design document should be written in any way that communicates the information effectively but is it ok to stray from what the document says? If you stray from the original plan should you update the document?
A GDD can NOT be rigid. A GDD is always and always will be unique.
Did you read the article I linked to? The point of a design log is to keep a record of your decisions and to help maintain a shared vision amongst team-members. It's not an approach that works for everyone -- and there are certainly good arguments in favour of the more traditional approach -- but it's certainly not pointless, and it's an approach that works well for some developers, particularly if they're taking an iterative approach that can be ill-suited to following a lengthy pre-written document.
If you're not going to follow the design doc putting it together is pointless.