I've never agreed with cages. They're usually used as a prison and people excuse this behaviour by using the 'safe place' claim they've heard about. A safe place is somewhere, anywhere, the dog associates with safety, not necessarily a cage. There are countless dogs in kennels that would really disagree that a cage is a safe place!
I'm not sure why a nice comfy bed where the cage is situated wouldn't be just as safe a place for the dog. I suspect it's more about someone controlling the dog without having any ability to teach it how to behave.
Igor (Mick Dempsey's) dog clearly considers the front seat of the pickup his safe place, as do others mentioned in the thread. I've had many dogs at work with me before as I fostered problem dogs for years, took the back seat out the pickup to make a dog area, every one I've known considered that a safe place, also their bed in the corner of the workshop was popular.
To my mind very few dogs thrive in an environment they spend 8 or 9hrs alone, but going everywhere with me, spending a few hours in their pickup safe place then getting a walk somewhere new, somewhere different, they enjoy that, and a pack is meant to live life together.
This thread has brought up some good memories of the arbdogs I've had. Some might remember old Santi, my ultimate arbdog, he accompanied me around the south of Scotland, parts of England, and a couple of times to France for work. His safe place was anywhere he could see me, either lying under the pickup watching me chipping or next to my bag when I climbed, or in the machine I was operating, and he appreciated most of all being rewarded for his patience by exploring in a different place after work every day. A cage on his own would've been so detrimental to him, not a safe place whatsoever.