Probably the most important issue this would resolve is what I commonly run into later in the game when it can literally take weeks for me to advance to the next level(Not because I'm so highly leveled, but because I usually don't play more than half an hour at a time... I know, I'm an undevoted sinner.)
When it gets to that point, and you want to expand your army, you have to wait for way too long to advance to the next level only to advance your charisma level by one point and add one more troop to your army. With the vassal system, and the implementation of allowing nearby parties join your battle once you initiate it, you can now add the armies that your NPC vassal is capable of commanding to your own in strategic calculations. It would also add a level of realism, as many battles have been won or lost depending on whether a commander's subordinates were following closely enough to participate in the battle in question.
Then we could get into ridiculously large battles... Which is exactly what I want...