Disclaimer: I am aware that this theory is very very likely incorrect, and am only posting it because I like the idea of Brandon trolling his readers.
Alrighty. *puts on aluminum hat* So this is the theory. Adolin will actually be the main character in the back half of the series. Brandon has purposefully said he is more interested in Renarin than Adolin to incite unhappiness and throw everyone off of the trail. He indeed does plan on having Renarin as a flashback character for the back half, but not in the way that anyone expected. One way or another Renarin will die in book 5, and the resulting incident will greatly affect Adolin. Perhaps he goes bad and kills Dalinar, and is killed by Adolin. Perhaps it is due to other circumstances. This is the reason that Brandon has discussed his willingness to kill off characters and still write their backstory is foreshadowing for this. What happens is Adolin goes to the Nightwatcher, and wishes to understand his brother. As a result he gets a chain of his memories. Curse is pending.
TL;DR: Renarin as one of the back 5 is a trap, indeed a bait and switch for fan favorite Adolin.
For sure.