First of all, 53rd week doesn't necessarily have to fall on Leap Years. It depends on company to company
In your case you have kept the setting in such a way that you are planning for 53rd Week in FY16 and from FY17 Onwards you will keep planning for 52 weeks until you cumulate another 7 days which will happen in FY22 and not FY21. Illustration as below
In that case, don't you think it's a disadvantage of the 4–4–5 calendar is that it has only 364 days (7 days x 52 weeks), meaning a 53rd week will need to be added every five or six years(not necessarily a leap year as in my case here.).
1 Functionally Its not a disadvantage. There are pros and cons to each method. Companies do plan at a weekly level and their 364 days Year keeps on changing the start date and end date on a yearly basis. In today's world where strategic or operational decisions keep on changing based on the macro economic or micro economic factors you have to adapt to such conditions quickly. If you ask me I would say Planning at the month level is easy, less complicated but at the same time you lose the visibility on your numbers on a weekly basis, react to the market conditions fast (but not faster)and you plan at higher level whereas weekly is a little bit complicated but allows you to plan at more granular level and you react to market conditions faster.
2. Technically too it is not a disadvantage - You won't have to add it manually, Anaplan will figure it out on its own based on the settings that you have on Time Page. Left over day of Year 1 will be planned for in Next Year Week 1 , so everyday is getting included in the plan its just a matter of classification.