Two questions about leaders.
1. What is the point in stacking leaders with your regular units? The only benefit you get is a small boost to KN/HC in their shock/charge attack roll. But it also makes the leader vulnerable and losing your leader is a BAD thing. If you instead leave them wandering around on their own (close by so the units are still in command) they appear to be invulnerable. You can't shock them. You can't shoot them. The worst you can do is move onto them, which just teleports them to the nearest friendly unit (which they could move away from on their next activation).
Your point is very valid and stacking leaders with units is something I rarely do. Your leaders can be surrounded and then eliminated if they are alone, but that is very difficult in practice. This is a design choice that lets the player decide to leave the leader with the unit or to have them wander alone.
2. 14.2 Disorder - If a disordered unit receives a disorder result (which it ignores), does that still trigger a leader casualty check (I think the answer to this is yes)?
Yes, per 5.5 it is the combat result that triggers the check, not the act of Disordering.