I tend to think that an obvious ring of rocks is less of a tripping hazard than a firepit without that boundary around it. Especially with kids stumbling around in all sorts of different hours of the day and lighting. Not to mention me stumbling around, and if I was going to fall over a rock I'd be a danger to myself going camping that's for sure.
I wasn't suggesting that rocks really do stop a fire from spreading, the only thing that guarantees that is our vigilance.
But I never keep red hot rocks in my lap so I've never had one explode there.
Rocks on the outside of a fire don't usually get hot enough to explode, or never have on my camping trips. When I was younger I learned that not clearing them out from under the fire before you light it can make for an interesting evening. That is easily avoided though. Nowadays teaching the kids to respect the fire is one of my main considerations when building and lighting it. And I feel much safer with that barrier between the fire and the kids.
Burnt ground is the first thing I look for when I arrive at a campsite and consider how I will shape it. I don't want to pitch my tent on the burnt patch, nor sit on it while I put my firepit in the grass so to me there's no question of where the fire will go. As for glass especially, and any other kind of rubbish people leave behind. grrr. What else can you say?