Looping 20, 30, 50, 100 in a row, its okay. Not important but not bad. It is kind of fun and good for cardio.
There are other things like multi-ball footwork drills that are good for cardio too.
Do a FH to FH loop rally 3 times, then you try and finish the point the quickest/best way you can, by...placement, varying your spins/speed, swerving the ball, angles etc.
Serve and receive drills are also good for learning to end the point.
Varying placements is very important. Even slight variations are very useful. Also, aiming for the opponent's switching point. There are three important angles to work on: crosscourt, opposite crosscourt and down the center.
Usually a player who is good at handling shots to his/her crossing point will not be as good with wide angles and a person good at wide angles will not be as good at shots to his/her body.
Some reasons why TT players do not reach 1500, 1800, 2000 levels.
....
I'll stop at 51 beofre any of you throw a frypan at me.
Another thing that holds people back is not watching the ball, really watching the ball; all the way into the other person's racket. This makes it so that you see the angle that your opponent's racket has on contact which tells you where the ball is going and what spin it has on it. Higher level players usually do this much better than lower level players. And this habit sets up several things. Your footwork improves because you know where the ball is going earlier. Your reading of spin improves because you see the spin earlier and then you see it in the arc and bounce of the ball because you see the spin from the contact of the rubber with the ball. This is important on serves as well. A good player will read the spin on the serve even if it is well disguised. Watching the ball.
Tonight I noticed that one thing that sometimes causes me to stop watching the ball all the way into the opponent's racket is that sometimes I inadvertently watch the follow-through of my stroke and if I am watching my racket as the ball is heading towards my opponent, then I am not watching the ball as it hits his racket.