Over time trees will become overgrown. Areas deprived of sunlight will die and the dead branches will become, not only, aesthetically displeasing, but unhealthy for the tree. It will use unnecessary energy to hold on to the dead branches that will also interfere with sunlight, wind, and rain passage though the canopy.
Also leaving a residential tree overgrown can be very dangerous and a potential liability for property damage. If dead limbs aren't removed by someone, they will eventually fall, especially during Oregon winter wind storms. Some dead limbs can weigh one hundred fifty pounds or more.