The basic defence bonus from carrying a shield can be seen as merely a passive bonus that results automatically from having a hard piece of wood held in front of your body. The extra bonus from forming a shieldwall (which requires much more conscious effort) is cancelled while stunned. You could argue that merely holding a shield in front of you also requires some conscious effort, but still …
On the other hand, basal defence skills, without a shield, do remain entirely intact while stunned, which admittedly also seems unrealistic. Fleeing characters, by contrast, have all of their attacking and defending skill totals (including any from forming a shieldwall) reduced by 30%, which does make sense.
In a way, it would make sense if being stunned (in addition to cancelling any shieldwall) reduced a combatant’s total melee and ranged defence by some amount, similarly to lower morale states or being trapped in nets or swamps. The only trouble with that is that orc young and berserkers can stun automatically, without even doing any damage. So maybe it would be possible for stunning to only reduce defence totals if the stunning blow does damage to hit points? I’m not sure …
As for the original issue (morale checks while stunned), I can see it being plausible that enough consciousness is retained to feel morale effects (and to keep holding a shield and weapon without making any fancy moves), but I raised the issue because I wasn’t sure whether it had been intended that way.
