Exercising in a manner that maintains fitness in all components reduces a person's risk of physical injury and mental burnout. People who maintain a high level of overall fitness will not suffer adverse consequences like extreme soreness or fatigue when they try something new.
Because their bodies can become very efficient in movements performed on a routine basis, individuals can actually reduce the amount of conditioning they get if they continually exercise in the same manner. This efficiency is excellent for the purposes of competition. However, if they want to do more than simply maintain their fitness level, they should develop their overall fitness through cross-training. Focusing on all fitness components in this manner will also decrease the risk of injury from overuse.
While it is important to practice the specific skills required to master a chosen sport, maintaining a high level of overall fitness will only come from a varied routine. This will allow them to work some muscle groups while others recover. They will be able to exercise more frequently and in a variety of settings and conditions. The mental refreshment that they will gain from engaging in these different exercises will ensure that they never become bored with their workouts.