In duellling the point is to settle the matter of honor. Taking excess time aiming is considered unsportsmanlike. Might explain the sight design.
So if both parties shoot wide, it's spit, cuss, then smile and go home with honors intact?
It all depends on the region, the time period, the offense(s) that started the affair, etc. Different places and time periods had different customs and there were many variations of a duel that could be agreed upon. Generally speaking in later periods pistols were more accurate than earlier on and closer to target pistols. Some duels had multiple reloads and exchanges of shots if reconciliation was not reached. In some cases you have to fire before a set countdown ends, sometimes shots were exchanged at a signal, etc. In a marching/walking duel, the duelists walked towards one another, and the man that fired first had to stop and wait to receive the other man's fire. The second man was allowed to walk all the way up to a predetermined spot if he so pleased though sometimes a time limit was placed on him.
Generally speaking purposely shooting to miss was considered a mark of foolishness. You shouldn't agree to shoot at one another unless the offense warrants actually doing so. If you shoot into the ground or obviously fire away, you could also be intentionally or unintentionally implying the other man is not worthy of being shot by you. Nonetheless, there are many duels on record in which combatants are known to have purposely missed.