Leg spikes ("spurs" or "gaffs") are harmful to trees. The only reason a tree worker should be wearing them is to climb a dead or dying tree to cut it down, and never when the tree is being trimmed.
Here is why this is so important: When a climber wears leg spikes, he digs them into the tree in order to take a step up. The spikes leave ugly holes through the bark — the perfect place for bacteria or insects to enter the tree and cause disease or damage. Sooner or later, if the tree is unable to close up the spike wounds, those holes could lead to the death of your tree.This is especially true with pine trees.
If you see a climber wearing leg spikes to prune your tree, you have hired a company that doesn't care about trees and is not following the professional guidelines and standards of the tree care industry. Stop the job immediately!