The seasonal ladder is designed so that the best strategy is to join before it starts and stay joined to it for the entire duration. It's designed this way so that there's no advantage to leaving/re-joining at key times to try and manipulate it.
As Beren says, ELO ratings are all calculated first and then bonuses are added afterwards for number of games played. Because it's done this way, there's no disadvantage for playing people who drop out.
WRONG.. It is entirely dependent on the ladder rating displayed. I am certain of it.
Isn't it funny how the people who word their arguments so strongly tend to be the ones who are incorrect?