They do not worth the repair cost imo.
There aren't many guys riding heavy horses. Well, as I tried riding a champion mamluk horse (with a style before stats principle), I find it less useful and more team-bump oriented than the destrier. As if the only good thing it can do was absorbing hits like a tank (was useful against HA). Destrier has better offensive capabilites than those heavy horses most of the time. As there are teammates blocking your way all the time, destrier's maneuverability can be much more useful to avoid bumping teammates and hitting the enemy when necessary. Destrier's charge damage is also higher than the heavy horses as it have a better speed.
Now heavy horses have their usefulness actually. They have much better defensive capabilities, but that's all of it. A cavalry, is class based on offense (exploiting the weak spots among the enemy lines), defensive cavalry is bound to be less successive than an offensive cavalry. Yet those heavy horses do cost much more. A plated charger costs nearly the 3 times of a destrier, but is actually less useful than a destrier.
I'm not saying heavy horses are bad, they have their place. Maybe they are more suited for a high agi build (dedicated bumper, maybe something like a build of 9-27), or a typical Horse xbow. I'm not really sure. Their cost should be high, we wouldn't want too many people using them (would ruin the fun probably).
But they should cost less! Not that much less, but a bit less actually. Their usefulness is not worth the 4k repair. Something between 2k-3k would be more suited.