Plasmafied or charcoaled enemies also never respawn.
I didn't know about this "bug" until recently, but as long as the frame rate stays decent, I don't count it as a problem; One thing I loved about the game Myth, was that bodyparts never faded, and you could literally stain the earth with piles and piles of slain foes ~and it stays there.
*I'd rather there be no respawns at all. :shrug:
See the series had something neat about it from day 1 ~No resurrection ~anything killed stayed dead. All of its contemporaries (or most of them) had a means to haul your buddy back to town and spring him from death; Fallout was a pleasant change, and the only 'respawns' were plausible ones, where you encounter "different" raiders, and animals on the open world areas. Since FO:NV is not shackled to the need for timeless/indefinite play, it needs no respawns at all.
(Though a few, like in dog dens or the NCR outposts could work nice)