Fleeing enemies would be realistic, yes, but since they would just get hunted down anyway, it wouldnt make much of a difference.
That entirely depends on how the game would handle tactical retreats by the enemy. First, let's assume they are actually good at retreating. They cast expeditious retreat, they hide and become invisible and move directly afterwards. They dash and push sacrificial lambs towards you. They create obstacles to cover their retreat...
The effects on your side: frustration, no experience points, no bodies to loot.
The effects on their side: they learned how dangerous you are, so when they try again they will attack fully buffed, in bigger numbers, and from a better position.
Yes, retreating enemies good be a very nice thing to have, especially on Tactician difficulty.