They fill a unique space in the troop trees, they're fast cavalry but heavily armored troops - always have a decent speed sword with short range for tight siege situations, and often have crossbows for fire support when not mounted.
I get the feeling their frequency of death if left to their own devices is a) their fast horse since they'll often hit the enemy line first, and b) the predominant lack of helmets. Given that they don't have ironflesh, a shot to the head is likely to be fatal to them. Their shields are strong but small and I find they often die to archer/xbow fire.