Gentlemen,
Having always been ok to go either way with the Weak ZOC optional rule, the more I've considered it and read about not just Napoleonic battles but all conflicts I now believe the Weak ZOC should be left off. My reasoning is as follows,

. Once a unit was committed and engaging an enemy force its ability to change its priority of attention/focus in moving away from that enemy was VERY restricted.
2. In our period it was almost impossible and unusual from my readings, to achieve this movement away from the engaged enemy without one force disengaging first (whether that be withdrawal, retreat or rout), or a greater threat appears to the enemy engaged appears and takes the that enemy's focus. Which forces the unit to break off so as to be able to re-focus (move), or break in retreat or rout.
3. This meant that multiple units/formations engaging a single unit/formation had the ability to fix and then turn a flank or commit weight of numbers. Likewise this goes up the chain where a fully engaged formation (Bde, Div, Corpse, Army) had to have reserves to deploy in order to out maneuverer or overwhelm the enemy force. Otherwise the battle was a simple affair of engage and wait until one unit/formation broke from the effects of battle (fire or melee combined with perceived "threat" - the threat levels I think are handled well with the threat values in the game engine).
4. The 'eye of god' ability we have with the game allows that refocus more so than was ever achievable for engaged units in the fog of battle. Thus that surprise or weight of number sis often countered much more effectively than in reality was possible. This extends the ability of a unit/force to stand and be effective a lot more than was actually the case and allows for a greater casualties total than was achieved before the unit/force broke.
5. With the Weak ZOC 'ON' (ticked) it allows units to move and react with a lot more flexibility than they actually were able to achieve. I believe this lack of flexibility once engaged in direct action against an enemy force was a very real combat restriction when formed bodies of troops have been engaged through the centuries. The move to light troops, irregulars, open order formation etc was an attempt to keep the enemy focus and engaged and thus restrict the formed unit's options (fixing them) whilst keeping the ability to have distance, flexibility and the ability to re-focus(move) without being tied to the engagement committed to.
6. Maneuver units/forces became tools to throw in as flanking threats or weight of numbers and that is where lies the options open to commanders, which of their forces to employ and where.
So in summary the Weak ZOC allows units and forces too much flexibility that was usual. Its effect is to extend the fighting abilities and casualties that a unit and formation was able to withstand and thus drags out a battle unrealistically. It penalizes a more flexible and well structured tactical stance, defence or offence. Just the situation many complain about.
Arguments for and against would be a good debate.