Opportunism. If the opportunity presents itself... better take it!
Unless you happen to be captured with empty hands, the ransom is generally payable for a moderate sum on the third turn (or is it third day?). Staying the full 7 days only happens if: you had 0 gold on hand, or just about, and your judge doesn't care enough about you to make deals with their judge, or you are a cheapskate, or you are a really wealthy noble carrying over a thousand gold and you really don't want that much dough going right to the enemy.
