Well, you gotta realize that the horse wasn't really required to carry all that heavy a load. The weight of a man decked out in full plate armor wasn't all that much more than one in a Norman coat of mail.

I'd say your typical destrier would carry a man of some eighty to one hundred and twenty kilograms (I'd say that's on average, the latter weight being a knight large by today's standards). Add to that the armor -- which weighed less than a soldier's pack nowadays -- and you get to a maximum of some one hundred and forty or so kilograms, and a minimum of some one hundred, one hundred and ten kilograms.

Now, how much does a fifteen-hand horse like the Friesian weigh? Two hundred, three hundred kilograms? More? Add its weight to its inherent strength and I'd say it would only have to carry something that felt like a quarter or a third of its weight around on the battle field, and I'm thinking that's a generous estimate.

So, no, I don't think these big work horses were war horses. They don't even gallop all that fast (take the Belgian for instance), being so bulky. And speed was an important part of the couched lance charge. If anything, these horses were stronger, faster and had more endurance than any step mount (hence the known fact that every Mongol had four to five horse with him).