So no doubt you see a wide variety of conditions.

On the boats on which I crew, we never send anyone aloft without a second safety line.

That said, on one unique occasion when a spinnaker jammed with the halyard right at the block, I saw a bowman ascend the mast without a harness, freehand, like a monkey. He was right at the trouble before we could even break out the harness. It was an amazing sight and yet, no doubt terribly unsafe. I would not have even though one could climb like that, but he grabbed a halyard, got up to the first spreader and basically acted like a monkey and made his way right on up, using halyard, spreaders, and shrouds to make the ascent.