Some of us find killing respawns, using different weapons or methods, a lot of fun...and because I can. An empty world would be a dull world. I still wish more deathclaws respawned....
I don't give a damn about 'immersion' when their corpses are marked persistent, it just becomes annoying.
Bodies always disappear in a couple days or so for me. Ash/good piles do not, tho. Another reason not to like energy weapons as they are.
What I find annoying is that Kings messenger who constantly runs up to you...I feel like he's there in my face, suddenly, every few minutes. He must be glitchy.