Why Topping Is Unacceptable
Topping trees is a common but completely unacceptable practice. According to the ISA, "Topping is a detrimental pruning practice performed on trees" which creates permanent negative effects. Among the long list of effects of topping include lasting health problems, structural integrity issues as well as permanent openings for water penetration and pest infiltration.
To thoroughly understand the depth of the damage done, first we must cover the tree biology that covers the process of healing damaged or cut branches. At the base of every branch, there is a layer of cells that constitute the visible swelling of the joint between the offshoot and the parent branch/trunk. This layer of cells is vital to the healing process. When the branch dies or is cut, this layer hardens and dries, sealing off the rest of the tree from outside moisture, infection, or pests.
When a branch is cut in the middle of it's growth, the branch collar is not stimulated enough to harden to protect from outer invasion. This results in vulnerability in the affected branch or branches, leaving the entire tree vulnerable to the devastating effects of the surrounding climate. Often, these effects can be fatal, either immediately, or over an indeterminate length of time. These effects may take a few years reveal themselves, typically in the form of dead ends or sparse foliage during the growing season.
However, in the case of long-term survival in which infection, pests, etc. do not kill the tree, the dense but springy growth that presents itself from the stub is not structurally sound. Not only will this growth be unattractive, it will be prone to splitting and breaking, either during a storm, or once it becomes unable to support the leverage imposed by the length and weight of a thin but long growth.
The reason many arborist will refuse to "stump" or "top" or otherwise cut a tree or branch NOT at a joint or "crotch," in addition to the aforementioned health issues, is that the stub will produce fast growing offshoots that will need cutting again by the next season, essentially negating the work done previously. This will create a cycle of stumping, trimming, breaking, and unaesthetic growth that will permanently affect the aesthetics of the entire tree
Long story short, topping is a permanent disfigurement with aesthetic and biological consequences. Don't do it, and don't let so-called arborists working on your property do it, unless you are intentionally ruining your trees.