Looming a horse has more benefits than a sheer increase in survivability, such as speed and manuever boosts.
My guess is that the loom bonuses make the difference of surviving an additional hit for lightly armored horses and up to two or three additional hits, given those hits were executed properly (glancing terribly is not an example of that).
On top of that, given you're a good rider; a loomed horse makes you avoid from getting hit much more compared to a non-loomed horse. This is especially true for lighter horses but this benefit is arguably less for "tanky" horses since that little difference in manuever does not make up for it in most cases.
That being said, my horse rarely gets hit when I choose to engage in melee with my +3 destrier; yet it offers enough tankyness to survive against projectiles. I think a good defence and offence for cavalry is manuever and speed, and a bit of armor on top of that to survive those few unexpected arrows.
