As the subject title says, I have a bit of trouble understanding why one should take mounted skirmishers instead of formed cavalry. -1 in firing at skirmishers might be a reason for survivability for fewer bases, but are there any other? Evading charging troops is a reason as well but if formed cavalry already have it like Breton retainers?
Should I pick mounted skirmishers to act as fast screening units to protect one's heavier cavalry from ranged fire? Or can they be used as skirmish hunters, as they may not charge formed troops but it says nothing against other skirmishers?
All in all, have I already answered my own questions? Have you had success with mounted Skirmishers? I think units equipped with bows like huns might fare better than those who got javelins or just spears, but you may have an answer that as well.