I thought only creatures "inside" didn't respawn.
There's a house in Nipton with a hostile Mr. Handy in it that respawns, but I don't know if that's unique.
Why would you leave 1 Deathclaw alive? I mean once you've looted the bodies of their power armor and other goodies it highly unlikely you're ever going to return to this spot.. I know in my game, now that I've done it - I'm never coming back - what's the point?
I guess to re-live the fun of fighting an army of deathclaws? If leaving one would ensure they respawned, that is, I'm not sure if it works that way though.