Insurance is fundamentally a numbers game, that's why actuaries get paid a lot of money.
At bottom, they charge you what they expect the cost of damage/loss to your tractor will be in the life of the policy, plus a bit for profit. On average buyers of insurance therefore lose money (the profit margin).
The real question is whether you're stupider/unluckier than the average insurance buyer, or smarter/luckier. If you're smarter/luckier, then on average you're better off self insuring.
However, there are some wrinkles in the calculations:
1. (and the most important) insurance turns a small chance of a big loss into a big chance of a small loss. For a lot of people that's smart business. They can afford the small loss that you'll most likely take, and they avoid a big loss that might seriously stretch their finances. That is the fundamental purpose of insurance - to spread the risk, and convert a risk of catastrophic loss into a certainty of a small cost. If you can't afford to self insure, then you should buy insurance. This is also why you shouldn't really be looking at whether you "came out ahead" on insurance. You're not supposed to come out ahead most of the time, insurance isn't for paying for day to day maintenance/repairs. It's for catastrophic loss, and if you don't get one of them you should be happy about it.
2. KTAC in particular has the repairs and parts provided by Kubota. I think that they're leveraging their ownership - what they pay to repair or replace a tractor is not what you'd pay.
3. They repair it different than you would. When I damage my tractor I fix it myself. A little panel beating and a rattle can of Kubota orange, and I'm OK with that. If you want your tractor fixed "like new" then KTAC is a good idea, if you're OK that bits and pieces of breakage happen and it's no big deal, then KTAC may not make as much sense.
In short, if you're a reasonably new tractor owner (aka likely to break things until you get the hang of it), and you like your tractor to remain in showroom condition, KTAC is likely to be good buying for you. You'll do more damage than the average person, and you'll get your tractor fixed pretty much like new.
If you're a relatively experienced operator who doesn't do too much damage (compared to the average punter, including businesses that have apprentices drive their tractors....), and you're OK that your tractor over time will accumulate battle scars, then you're probably better holding insurance only for total loss/fire and the like - i.e. homeowners insurance.