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. If you see a climber wearing leg spikes to prune your tree, you have hired a company that doesn't care about your trees and is not following the professional guidelines and standards of the tree care industry. Stop the job!
Here is why this is so important: Do you see the sharp spikes on the inside of these spurs? When a climber wears these, he digs the spikes into the tree in order to take a step up. The spikes leave 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 have hired a tree service to prune your trees, tell them ahead of time that you will not tolerate the use of leg spikes. Then, if you see someone in your tree wearing spikes, it is your right to discontinue the job and either fire the tree service or insist that they climb the tree using ropes.