Have you already bought every single Telltale episode released? (TTTexas Holdem, Bone, Sam and Max, SBCG4AP, Wallace & Gromit, Tales of Monkey Island)
If you have... then yeah, the coupon might be wasted. Or you could give it to a friend? Or give it away on the forums for someone else to use...
(I now have bought all of Telltale's games - some with coupons - and thus any more free codes I get will have to be given away....)