It can be sublimely helpful... It can also be wicked dangerous.
I've lashed out in my sleep more times than I can count, and I've come awake -still in a nightmare- in response to external stimuli. Conversely, sleeping with combat types, I've also woken up being part of someone else's nightmare. Thank god for partial sleep paralysis, or my throat would have been crushed on a few occasions, not that it didn't hurt but could have definitely been worse. I've also gotten some stellar bruises as well as a broken nose to add to my collection from knees, elbows, fists, and one head butt. Not how I like to wake up, but sudden pain is better than sudden cessation of oxygen. Especially since peeling hands and forearms away enough to talk usually results in bruises, anyway. Dude! Wake the f*ck up! I can't breathe, here. You soooooo owe me dinner for a week. And I'm not doing dishes this month, period. C'mon. Help me set this. Sigh. My poor battered nose. All stunted and lumpy. But they always mean to hurt me as much as I mean to hurt them... Aka not at all.
Doesn't mean it's not still dangerous, though, just cause it's not meant. I have a firm rule of nothing that can be used as a weapon (no lamps, cords, pens, bedside tables, etc.) within arms reach when I'm in a bad place and sleeping in the same bed with anyone.
The upside to sleeping with someone is that they do nix about 99.9 bar % of my nightmares. The sublimely helpful part. The downside is the risk of my hurting them in my sleep, or being woken badly.