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A few thoughts to consider...
* New content in the new version of the PMBOK is usually not brand new information that nobody has heard of. It's more likely information that has been getting attention in the year or two prior to release.
* There may have been questions related to this information on the latest exam, before the new release.
* 100% of the PMBOK may not be applicable where you work as a project manager. I know, this sounds like heresy. But be honest; does every project you run have entries on the project schedule that address activities and deliverables from every process group and knowledge area in the PMBOK Guide? (Hint: it's called a Guide for a reason)
* A project manager that can't stay current on the things that matter in his or her job market will, effectively, remove him or herself from the job market.
My cynical (realistic?) side wants me to point out that a training company is going to create hype and urgency around changes to the exam because the training company is a business. To stay in business, you need customers. If there is no urgency for a good or service, you won't have many customers.
To answer the follow up question, there is not enough data to definitively state whether or not 20 hours per year is enough to stay current. It might be for some, and might not be for others.
If I were going to invest in 20 hours of training, it would, first, be on topics that directly relate to upcoming projects that I need more information on, and, second, on other topics that interest me. New material in the PMBOK Guide might not fit into either of these categories, for me.
You should also consider that the people actively engaged in forums, like this, are more than likely doing more than the minimum to maintain their credential, and doing more than just training classes to accomplish this. Someone that relies solely on training might be at a disadvantage.
