As arborists we should be concerned with minimizing injury to a tree.
A tree is alive just underneath the bark. This is where the vascular cambium is and really the only ‘live’ part of the tree. Spiking (spurring, hooking, gaffing, etc) up a live tree causes multiple injuries to the phloem, cambium and xylem and destroys the tree’s own protection against outside invaders, such as insects and other pathogens. For example, it seems a healthy fir or pine tree with no injuries is less attractive to sapsuckers and borers than a sick bleeding one. Make a couple of holes in that same healthy tree and you will undoubtedly attract more sapsuckers and borers. They can smell/sense the sap and therefore will target the tree.
Some research has also shown that infected climbing spikes can transfer some disease organisms from diseased trees to healthy trees, much the same way the common cold can be spread from person-to-person.
A tree must also use energy to recover from a wound much the same as an animal or human would. The overall vigor of the tree is reduced since energy that would normally be used for other normal metabolic functions must be redirected to heal wounds. The tree will probably survive but overall will not be as vigorous as it would have been had it not been injured. Spike holes also provide entry for bacteria and fungi which could infect the tree, possibly leading to its death. Hiring a skilled arborist that has the correct equipment and knowledge is the best step towards preserving your trees when pruning.
In addition to health concerns for the trees, the holes that climbing spikes leave are just plain unsightly and can diminish the value of your trees!
These are sad examples of a fir tree that was spiked by someone from a door-to-door tree company when “wind sailing”: