I have a few friends lose multiple Dura Tracs to sidewall cuts, but that one doesn't look too bad. Do you have five DTs or only four? If only four, and you have the cash, sure, buy another and make the nicked tire your spare.
i had some pretty deep cuts in my BFG AT. they held air for years that way and never a problem. i finally split the worst of the slices open when i dropped down into a big rut at considerable speed. it finally gave out. i got rid of that tire, kept the other 3 ones that hold air for spares, and got a new set. my gashes were MUCH MUCH worse than the one pictured above. you should have no problems with that.
If you do decide to keep the tire as a spare I would recommend that you seal the split up with Shoe-Goo. That way you will at least keep dirt and moisture out of the inner tire.
From a road worthy point of view this tire should not be used on road. The integrity of the side wall may have been compromised and could result in a blow out, there is no real way of finding out how much internal damage has already been done short of dissecting the tire. Of course the decision is yours, you have to weigh up the cost of the replacement tire against the "cargo" you carry in the vehicle.
I put the tire with the small cut in the spare location. Peace of mind is better than blowing out a tire doing 70mph...plus I have read that the negative of the DuraTrac is the sidewall.