Wouldn't it get a bit boring if everyone kept running away from you? It is a game after all. The bandits are there to be killed not chased all over the countryside.
Well, if they realised that they didn't want to attack you, they might just not attack you - you would be walking around and there would be a pair of armed men hanging around the side of the road, not attacking you, maybe even showing off some new idle character animations/conversations.
For the characters who realise they're about to get their asses handed to them, if they're that close to death, they should limp (like in Fallout 3) and should be easy to track down. My favourite thing about being an expert archer was picking people off in one shot - that should be the end of the battle, picking off the half dead bandit limping/crawling away.
@ Swedish berzerker:
At the same time, it's still a video game and rats, bears and wolves are there to be attacked. Realism only needs to go so far. If it were realistic you probably would see about three wolves or bears during the entire game. This doesn't need to be Cabella's Hunt IMO. We still need some mindless battles from time to time. Still, I like finding the wolf packs in Nehrim. hopefully Skyrim can respawn a wolf a few times to make it seem like a pack.