Besides being good TV, would be a perfect red-herring for non-readers to speculate over when Joffrey dies. If the writers are really on top of it...they will keep the Joffrey/Tywin dynamic going strong. And then they could build up a Joffrey vs Tyrells...that way when Joffrey dies, the non-readers could wonder...was it the Tyrells? Tywin? Sansa & Tyrion?
The speculation would be great. Especially since the non-readers are already speculating that Jon Snow is Rhaegar & Lyanna's child and not Ned's kid.
~Jeordam
Saving the Princess, Humanity, or the World-Entire since 1985