When you top a tree you introduce stress. You remove 50-100 percent of the leaf bearing crown, a tree's only food source. The result is that the tree needs to find the energy to put out a new set of leaves or buds ( when the tree is dormant).
The pruning wounds that result are large and open the tree to disease and infection. Exposed wood begins to decay and those decay organisms are given a free path to move down through the branches causing damage to the rest of the tree.
The new shoots that appear after a tree is topped are not well anchored and are very prone to breakage requiring expensive future care.
Topping when the tree is fully leaved out exposes the remaining branches to high levels of light and heat. This results in sunburn and consequent damage to the tissues beneath the bark, further weakening the tree.
Topping is expensive. 1) Since the tree is now considerably weakened a topped tree will require more corrective pruning in the future and may die requiring costly removal. 2) Damage caused by branch failure may lead to a finding of negligence. 3) The resulting tree is ugly and reduces the 10-20% increase in property value that would have been achieved by a healthy tree.
Beware of anyone who tells you that topping a tree is ok. While it was once a common tree pruning technique, we know today that it is a bad idea.
To read more about google "tree topping" or check out http://www.treesaregood.com/treecare/resources/WhyToppingHurts.pdf.