I let others ride my horse but only those that are better riders than I am who I know and trust.
My friend where I board my horse is allowed to ride him when she wants (only took me up on it 3-4 times though) as she is a very good rider, he knows her, she knows him, and we agree on riding styles, etc.
My riding instructor will often ride him for 5-10 minutes at each lesson to help me figure out his cues, what he knows, doesn't know, etc. as I am a beginning rider.
I will let my girls ride my guy but only in a round pen with an adult there. He is good but fast so I want them closely supervised.
I don't let just anyone ride him. I would be very afraid of something happening, someone jerking on his mouth, etc.
I did let my riding instructor's 14dd ride him 2 weeks ago to run him and see how he did. She is a very good rider---but I still made her wear a helmet on my horse