The problem of the courser is that it is a giant "hit me" sign. It's actually comparable, if not bigger in size than the destrier, and a lot more squishy. You have the speed, but not the maneuvrability that would naturally come with it. It's the hardest horse to control and you'll crash into pikes or other pointy things often.
I think the reason many choose the destrier is it's really a median horse in all categories. It doesn't drop dead after two shots, it doesn't tear your wallet apart, it only needs 5 riding and it goes and turns barely fast enough. I believe many actually prefer the arab warhorse but don't want to gimp their build with 21 agi, or at least think it would gimp their build.
BTW. Does it still stand true that loomed courser has less hp than unloomed one?
My +3 arab has 15 more hp than a normal one. I guess it's roughly the same for all horses.
This is the build I'm planning to follow (currently level 27), using +3 arab, +3 KAS, +1 HL and an elite cav shield. I'm already satisfied with 5PS, even on foot, as I hardly ever bounce. The 4 shield skill also seems enough.
Level 30 (4 420 577 xp)
Strength: 15
Agility: 21
Hit points: 50
Skills to attributes: 2
Ironflesh: 0
Power Strike: 5
Shield: 4
Athletics: 7
Riding: 7
Horse Archery: 0
Power Draw: 0
Power Throw: 0
Weapon Master: 6
One Handed: 145
Two Handed: 1
Polearm: 72
Archery: 1
Crossbow: 1
Throwing: 1
I usually try to dismount/kill a few enemy cav when the round begins then when things start to settle down I go ninja on the occasional unaware ranged/infantry on the ground. The arab horse is the best for taking on other cav and killing those damned bazooka fairies (when they don't kill the horse first).