I find longer weapons can be great team support in melee, they're just not really viable in solo combat.
Hegemon, people do throw things from horses, and it works.
The reason all mounted troops tend to use spears and lances is because that's what they're equipped with by default, and using anything else leaves you at a deadly range disadvantage anyway. I can't speak about glaives on foot having not used them yet, but I assume they're mainly for anti-cav.