Well yes... but for me the jump from champion rouncey (never had the stomach to ride the silly sarranid horse, doesn't fit the western-type horseman theme and I think they're a bit nonsense / op) to champion destrier, it really makes a difference in survivability and what you can do in general.
Champ. rouncey was decent for horse crossbowing though since there you don't have to come in direct contact, but for longsword from horseback having a good horse really makes a difference since you just have to be exposed for longer, and you sometimes have to knock down infantry which the rouncey is not good at - it's liable to get stuck in people.
A champion warhorse would probably be even better, but even a champion destrier with heraldric mail gear forces you to spend some rounds on foot (then again, on many maps you're better off on foot anyway) to make money.