Certain abilities from books in later 3E came about after designers finally realized that most people looking for "fighting defensively" options probably had Combat Expertise, and had specific text saying, "when you fight defensively or take a penalty of X with Combat Expertise..."
The value of X varied, but IIRC most often the magic number seemed to be 4. So, the answer to your second question is more accurately, "sometimes" (when explicitly stated).
EDIT: In the case of Defensive Opportunist, no. It doesn't specify Expertise, so it would only let you ignore the -4 penalty from fighting defensively.