Proper pruning techniques are discussed in detail in this Texas A&M article.

Topped trees form watersprouts which are epicormic branches without structural strength. The branches have weak unions and will break easily and the ones on the ash trees above could fall onto walkways and streets causing damage and injury. Also the sudden opening of the lower branches to the sun can cause severe sunscald damaging the bark and further weakening the distressed trees. Flowering of the crape myrtle will be reduced since the energy of the tree will be directed to shoot growth and emergency leaf growth.

Links to further discussions of the harmful effects of topping:

Crape myrtle: note the long un-topped stems with u-shaped crotches and many flowering branches

Rio Grande Ash: note the long un-topped stems- all stems exhibiting strong leadership to the tips-strong U shaped branch unions that are strong and safe for passing pedestrians.