It is basically and insurance policy that is great to have when you need it. I payed for it when I put tires on my truck the last time because I caught a chunk of brass off the road and ruined a $270 tire. Had to buy two tires for the grand total of ~$700.00 while on vacation, ouch.
My rule of thumb is if the cost of the total free replacement fee is equal to or less than half the cost of one tire $22.00 X 4 (I don't have a dually) then I think it is worth it. Chances are not great that you will ruin a tire but the one thing you know for sure is, if you do ruin a tire it will cost you at least $270.00 minus the prorate. In my case if I ruin a tire I save ~ $200.00 and if not I'm only out $88.00.
One mans opinion,
Hope this info helps!