On more point on Light Cav -- they can actually chase down a routing enemy general, where most heavy cav cannot. This is strategically important on many levels: higher ransom potential, you won't have to fight the same general again in a different stack, and also super important if facing a Crusade/Jihad army.

Also, LC tend to cost substantially less in upkeep than an HC unit... so why waste money every turn with extra HC units where LC will do? They work very effectively at a lot of the uses people listed above: harrassing missle troops, quickly moving around the flanks, attacking pinned spearmen, fighting missle cav, etc, etc.