Spurs cause a specific type of damage to the tree whereby every spike hole pierces through the cambium layer of the tree causing wood decay and an entryway into the tree for insects and diseases. Climbers, ones that I've used anyway, tend to scuff up bark more but the damage is primarily limited to the outer bark which may look like more damage has been done to the tree standing on the ground looking at it, but physiologically doesn't hurt the tree. Look up cork harvesting. Visually it looks like massive damage is being done to the trees but doing it in the proper way, the tree survives it fine because it's done carefully to only cut the non-vital portions of the tree. Cut a little too deep and it girdles the tree and kills it.
Tree biology is a little tricky to understand. I can cut off say a 4" branch in the proper location, just outside the branch collar, and the tree relatively quickly compartmentalizes the decay and calluses over the wound in a few years. Make the same cut an inch closer to the trunk cutting into the branch collar and it can lead to decay cavities in the trunk and take many years to callus over. Same size cut, slightly different location, very different damage to the tree.