Fighting defensively is a simple -4 attack for a +2 Dodge bonus. The rules for fighting defensively are the same for standard actions and full attacks. Pounce allows full attack with charge. I don't see the confusion.
For me, the situation would be if you charged and took unexpected damage from an attack of opportunity or something along the course of the charge, and then you wanted to fight defensively. That seems reasonable. Of course, the other reason is you just want to close the distance for whatever situational reasons. So the charge with fighting defensively could just be the most appropriate tactic -- block the portcullis closing but don't get killed by the guards, stop that archer/wizard and hold on till the rest of us get there -- that sort of situation.