Nobody is good enough to only play offense and never play any defense. Nobody, period! Defensive astuteness is a very real skill that needs to be attended to and developed. It makes logical sense if somebody is on offense, inevitably somebody must be on defense. Against a very good player, evenly matched with yourself, there is a very good chance you will be spending half of the match on the defensive end playing hockey goalie.
Below is a slideshow of all the top pros on the run (dead sprint!), in nasty, uncomfortable, and mind-boggling positions. Yet they are fighting like sunsabitches to weasel their way back into the point. Many juniors fall into the trap of only doing drills that are warm and fuzzy, working on their technique, minimizing the movement. Ahem, crosscourts! Ahem, feeding drills!
On a funny and serious note, here is Herm Edwards, the infamous NY Jets and Kansas City Chiefs Coach talking to reporters after another poor performance. "Hello!? You play to win the game!" Yes, in tennis, you play to win. Technique goes out the door when you are sprinting 20mph...backwards....into the corner...snot up your nose...opponent on top of the net...switching your grip to find the right angle...sliding into the shot...breathing heavily...odds stacked impossibly against you...diving into the shot...PLAY TO WIN! Perfection is out the door, you play to win!