No matter how you look at it, i think pruning with spikes is detrimental to the health of the tree,
We must remember the only real growing part of the tree is the cambium directly underneath the bark, even if where spiking thick bark the likelyhood of hitting cambium is high because its only just under the surface of the bark, hence the reason why bark is so thick on some trees, perhaps they require more protection?
We had a few pine trees we spiked at college, bark was thick but there is definately signs of dieback over a few years spiking.
What do you think?
