First, my horse isn't suitable for beginners. He might not have the sharpest reactions, but he doesn't react well to imprecise cues or ones that are too hard on him.
Second, I don't see a reason to. He is not a tool for entertainment, he is not a lesson horse. Why would I let somebody just ride him for kicks and giggles? Go to a lesson or trail ride facility, if you want to, but the fact I have a horse doesn't mean that I'm obliged to give rides to people or that he's obliged to carry anyone who'd jump on him.
Now, I have let my trainer to ride him for obvious reasons - he benefited from a more professional rider and I benefited from observing his learning process, and for such a rider he behaved beautifully. I have also ponied my best friend and my brother on him at a walk, and he was just fine. But that's something I do rarely and not something I perceive to be a needed part of my horses life.
I have come a long way, to surrender my shadow to the shadow of my horse.