The benefit is that you can cancel any time you want, and are only committed on a month-to-month basis. This might serve as a good move for Amazon, allowing people to dip their toes into the Prime water before upgrading to the yearly plan. It also serves to underscore how great a deal Prime is.

But if you are already familiar with Prime, these new plans only really make sense if you see yourself canceling in the near future. 19% of Prime's current subscriber base has canceled in the last year, according to research by Parks Associates.

From the article "Amazon's New Netflix Competitor Is A Bad Deal For Most People" by Nathan McAlone.