From another point of view, a given date gradually occurring on every day of the week is a feature, not a bug. I see that the proposed calendar puts my birthday on a weekend, every year year after year, but I kind of like my birthday (and my wedding anniversary, and other important dates) occurring on a new day of the week each year, for variety.
I note too that the calendar would not "make it easy to plan annual activities" for any of the dates in the "extra week" proposed for the calendar, if by annual activities we mean activities that occur in every calendar year. The extra week occurs "at the end of December every five or six years," and the new memory load everyone would have to bear is remembering which years--2015, 2020, 2026, 2032, 2037, 2043, 2048, 2054, 2060, 2065, 2071, 2076, 2082, 2088, 2093, 2099, 2105, "et cetera" are the extra week years.
that "new memory load" is functionally zero, since no one would have to remember that. By the time the Extra week actually arrived, people would have been looking forward to it for an entire year, and it would not catch anyone by surprise. No memory required. Even subterranean cave trolls would be aware it is coming.
I think it's a nice idea especially if we could have a big weeklong worldwide party once every 5 or 6 years