Actually, I'm positively surprised at how durable the heavy cavalry horses are, and not just because the AI conveniently doesn't gun them down on purpose...
I'd also suppose that adding in higher charge damage would make cavalry charges against pikes too effective - the AI will almost exclusively target the first couple of horses, the rest could then ride down the pikemen at their leisure...
That said, heavy troop horses and heavy thoroughbreds do a decent enough job of not just knocking infantry about, but hurting them in the process - not that it matters much, as you shouldn't be charging heavy infantry/pikes (most of your cavalry carries firearms for dealing with those), but either enemy horse or shot. And just the slightest knock is sufficient to ruin a musketeers day...