We often hear parents or coaches tell their students or children to "just go out there and gave fun". Unfortunately, most players and coaches do not practice with this same goal in mind. Often times the too parties spend an inordinate amount of time developing the "meat and potatoes" of the game (forehand, backhand, some serves and volleys) while completely ignoring the rest of the game. Just like a mechanic has hundreds of tools in his box, a tennis player needs to be proficient at shots even though s/he may only be required to hit them a few times during the match.
For example, slices are not just great for defending, but they also work for neutralizing the opponent's weapons, for moving the ball around while not giving the opponent too much time (due to trajectory of the ball), for changing up the spins that you impart on the ball, as well as for breaking down the opponent's legs (by having her bend her knees repeatedly). In addition, swinging volleys are great for taking "moon balls" out of the air and not allowing the opponent to recover from a defensive position. Drop shots work wonderfully against players who spend a great deal of time close to the back fence. Top spin lobs, jump shots, sky-hook overheads, "lefty" forehands, mid-court chips, backhand overheads, "pick-ups" or half-volleys, between the legs, behind the back, etc. are all shots that a player should learn as you never know when you will be called upon to execute such shots at a critical point. Furthermore, executing these shots will give the player "true confidence"; the knowledge that s/he can handle whatever the opponent throws his/her way and that s/he can create openings with all of her tools.
Now, we're not advocating utilizing these shots exclusively; far from it. The whole point of practicing them is to understand not just HOW to hit them but also WHEN to hit them (and when not to do so). For example, a player who has executed 1000 drop shots in practice and has been successful at hitting 200 targets is less likely to hit them at an inopportune time (because s/he understands the risks and his/her limitations). But, again, it is important for the player to have all these pieces in his/her arsenal in order for the player to be able to truly "have fun." There's only so much fun that a player can have while hitting 10,000 forehands in a match. Lastly, a varied practice is more likely to be exciting for the player - keeping him/her coming back and asking for more - and result in LESS burn-out. So go out there are work on everything and use the tennis court as your canvas for producing great art.