The title is a little provocative, but how often are they actually useful?
Only once in 4 tyre failures would a boot have helped, when I nicked the sidewall on a cattle grid with a rough weld, making a hole just big enough for a tube to bulge through. That time we improvised with a tyre patch and duct tape, well enough to finish the ride. All my other tyre failures have blown the sidewall out at the bead, leaving a gap too close to the edge to boot, and longer than the boots I carry. Once I had a few km to walk, once I had a spare folding tyre, and on Saturday, having forgotten to pack my spare tyre, I used a boot plus duct tape, inflated very little, and just about managed to ride to a cafe 4km away (there was also a bike shop so I could have bought a tyre but we bailed anyway as we couldn't warm up). The boot did less good than the duct tape, as it didn't really stick to the tyre.
The anecdata I've collected fits with my own experience - when you need them, they don't normally help much.
So if I'm going far from home, I tend to carry a tyre. Boots are supposed to be get-you-home repairs, and that can be a long way: it's not unknown for me to be 150km from home riding through the night with the shops shut. Given that, about the only good thing to be said for boots is that they're small and cheap.
Am I missing something? Are they perhaps more useful on weaker tyres where a cut in the tread could be more severe? Or on MTB tyres (for those few of us who still run tubes in our MTBs)?