On the subject of 'extended warranties' in general I am in full agreement with you and do not buy them. However, in the case of tires, and really tires that are intended to be deliberately abused when offroading by way of repeatedly airing down, subjecting sidewalls to driving in steep ditches or ravines, etc, impacting rocks, etc, the Discount Tire 'certificate' is really more of a 'tire insurance policy' than an extended warranty. In writing they claim 3 years or 3/32" tread, but in reality its more like 10 years, they just don't put that part in writing...
In my table the total certificates cost for five tires ($243.75) is shown as a function of the total cost of a set of five tires and ranges from 10% for the largest and most expensive tires to a about 14% for the smallest, cheapest set of tires. And, yes, that averages out about 75% of the replacement cost of a single tire. Meaning, if over the term of the life of the tire insurance policy you get one tire replaced, you will have effectively gotten it for about 25% off the replacement cost (and perhaps more of a discount if accounting for time and inflation...).
In the case of my current set of five Nitto Terra Grapplers from 2017 I have had Discount Tire replace two of the tires, no questions asked, one with a offroading induced sidewall tear from bead to tread two years ago, and one right now with a sidewall bubble (possibly induced by years of airing down and accompanying side wall flexing). So, two tires replaced for free out of five originals that are 6+ years old with a lot of tread life left on the three remaining originals. To me the certificates were a good investment (especially since the previous owner of the tires and wheels paid for those certs...). Which brings me to another good thing about Discount Tire certificates, that being if you sell your tires, those certificates are transferable to new owner; which can increase the resale value of your tires...
To be sure, some (maybe most) people will never recover an investment in tire insurance. For a daily driver car with street tires that rack up lots of relatively benign highway or commuter miles where road hazard damage is pretty unlikely, the investment may not make sense. Likewise for a mixed use daily driver vehicle that racks of lots of highway miles and only a little offroad for which the tires might last only a couple years before the tread is gone, the economics might not work out... But for the non-daily-driver vehicle that really only gets 5-10K miles per year put on it and tires intended to be abused; based on my experience thus far, heck yeah, I'll take that bet.
I've often wondered about the concept of "can I just buy one certificate for the one tire that might fail"... And yeah, I can see how Discount Tire wouldn't go for that. As with any insurance (or gambling) it's a game of odds and actuarial science, and Discount wouldn't be selling the certificates if they didn't make money on it overall. Of course, when you get a tire replaced with a certificate they do push you to pay for a new certificate for the new tire (about $48). I presume you could decline on that certificate, and perhaps if that particular tire failed in the future they might decline to replace it and could probably identify it based on its date code relative to the date codes on the other tires with certificates.