Smoke 'em if you got 'em

Old movies with soldiers often saw drill sergeants relieve their squads with a jovial "at ease smoke em if you got em".

The same can be said for affirmative defenses.  If you have them as a defendant, then you should use them. Like the cigarettes in those old movies, you might not get to use them if you get killed in a battle.

For litigators it's more figurative.  But the civil rules of civil procedure in some state and all federal trial courts tell defendants that if they fail to raise affirmative defenses (with some substance thank you Iqbal and Twombley) then those defenses are waived and barred from later use.

Some lawyers may feel that they can "reserve the right to add defenses" in their answers.  Don't bet on it.  There is no such right to reserve.  The civil rules and the case law are fairly clear on this point.  Aggressive and unyielding plaintiffs' counsel will not stand by while affirmative defenses like good faith error, or acting on counsel's advice, or the plaintiff was exempt from overtime coverage because they were not performing the duties of an employee are raised well after the answer is filed.

Factually unsupported affirmative defenses are now just as troublesome as absent defenses. Thanks to the US Supreme Court's decisions in the Iqbal and Twombley cases (search those names and the cases will appear in google or any other halfway competent search engine) a formulaic recitation of elements of a claim or defense will be vulnerable to a motion to strike.

Use em if you got em.  If you don't, then expect my motion for judgment on the pleadings or a tough row to hoe at summary judgment and trial when your defenses are long buried by waiver.