I've seen this statement before but never understood it.
Can you explain why it's dangerous for someone else to clear your weapon? I'm assuming these people are knowledgeable on how to do it, not just some rookies.
There are lots of reasons, but you already named the all time big one.
The First Rule of Firearm Safety = Never Assume Anything When Loaded Firearms Are Involved.
Personally, I don't like anyone handing me a loaded weapon. It goes directly against everything I've ever been taught. Hand it too me mag out, bolt locked open, & chamber clear, or I won't take it. Not a pet peeve or anything. In The Marine Corps, if you present a loaded weapon to an Officer for inspection, you go straight to the Brig. Big no no. Very bad JuJu. It's been awhile, but I doubt they've changed that.