Respawnable one's don't bother me that much, but one's in cities and towns really piss me off. It's just relentless, every time you take a carriage to a small city you just know an Ancient Dragon is going to spawn and attack, then you get the moronic NPC's attempting to fight it in no armor and with their bare fists. I've heard Dawnguard just makes the whole game world empty after a certain amount of time, which is really causing my to hesitate before purchasing it.
Sure the peddler is respawnable, but he always spawns with the bandits so he is basically a walking corpse.
But I know what you mean. In one of my games the female blacksmith in Whiterun disappeared on me. Not sure if someone killed her or if it was a glitch. No dragons had spawned inside the City so I don't know who could have killed her and I have yet to find her body (and I have looked plenty). But she is gone and it is annoying not to be able to sell stuff to her. She is such a convenient merchant, since you don't have to go inside to deal with her and she is right in the beginning of town.