Summer and winter fruit trees and vegetables grow abundantly with unobsructed sun light. This extra produce can be shared with neighbors. These are ecological reasons for trimming trees.
Another reason is for safety of the neighborhood. It is important to be able to see what is happening on one's street. "Who is coming up the walk?" "What is that unusual noise in the middle of the night?"
These are only a couple of reasons why people should keep their trees well trimmed, preferably year round.