Questing around Whiterun, I was wondering if the many caves and bandit hideouts are later recycled for different quests, or if they always contain the same quest and once cleared will be empty forever.

Good question. I returned to a place I had cleared BEFORE even knowing that it would be a quest site later on.
Upon returning there during the quest, all the creatures I had killed were still lying around but the treasure had respawned (was necessary for the quest completion to open the chest and retrieve an item).
Weirdness.
My guess is that they do NOT respawn and that this just happened because I had gotten to the cave before activating the quest *shrug*

yes i did a quest where i cleared out and i had another quest to find an item and the dead bodies were still there but a different group of people were in this cave as well.

They do respawn, it just takes a while