I am quite surprised that your post has gone unnoticed.
Here are some of my thoughts:
It'd also be good if mercenaries made more informed decisions about joining AI factions.
Mercenary parties are also typically too small to stand up to enemy lords alone and spend most of their time raiding villages then getting captured when someone shows up to defend.
I agree with you. I feel that mercenaries should serve a specific purpose. For example, the Beni Zilal should serve as a raiding force for employing factions, rather than joining main army and getting destroyed.
I feel that mercenaries should be divided into a few categories, and I can think of raiders, soldiers, and vanguards. Raiders will have the predisposition of raiding villages and caravans, soldiers will join armies and increase numbers, vanguards should stay back and destroy raiders and join in castle/city defenses. When you employ them, you can tell them what to do, or else they will stick to their default dispositions.
While they may begin the campaign with their unique troops, they quite quickly become diluted with the recruits of whatever faction they are currently serving.
I once posted here as well that mercenaries should have their own strongholds, where they can replenish their unique troops quickly. I also think that they should have simple quests for you to build relationship with them so that you are allowed to access their stronghold and recruit unique troops, trade unique items, and forge unique items.