Nightmares are beneficial? How exactly? How is illogical fear beneficial? It kills mind, therefore it is detrimental.
I don't remember last time I had bad dream... What was my worst nightmare I had? Someone would maybe say that being shot or cut by knife to death is a nightmare, and while I had that maybe 3 times through my life, it wasn't nightmarish for me. When I was shot down I actually make it to LD, I was just tired of dying and it occurred to me that I don't need... I simply stood and walked away through wall. And that made aware of dreaming. I most dislike dreams where there is chaos and lack of peace.

I'm training relaxation and concentration directed for mastering OBE from age of 12(I'm now 41). I created my own system out of autogenic training(I found in sci fi book Vladimir Paral: Valka s mnohozviretem, (War with manybeast)), found my own way... remembered and reproduced feelings, states of mind... I do meditations for nearly the same period of time(it came spontaneously with exercises)... Maybe that makes one immune to nightmares.

I seek for inner peace, for control of illogical impulses... I seek knowledge. I think, it is why my dreams are not something I fear. But I would exchange normal dreams for lucid one entirely if I would know how to do it. Lucid dreams are more ordered... Greater, stronger lucidity is for me better way to live than let it slip into imagination where I could get lost.