This is time well spent! As mentioned by some others in this thread, idling teaches you some other skills. You will learn momentary still-stands. If you can practice large idles, moving from the 3:00/9:00 to 9:00/3:00 positions, you’ll be in a good spot to practice small hops during the still stands. You will most likely learn how to ride backwards after learning to idle.
I think there is certain value, albeit limited value, to practicing idling at a wall. It will help you learn how to exert back pressure on the pedals, feel the range of motion of your legs and practice changing directions. However, learning to idle involves a lot of uncontrolled right-left twisting, which a wall might inhibit you from doing.
I suggest trying the following method for learning to idle:
Ride forward, come to a momentary still-stand, then attempt to continue riding forward. As you improve, practice riding at greater speed–into the still stand. It will be necessary, under these conditions, to get the wheel out in front of you, prior to the still stand, with your upper body then swinging forward into a balance position. At some point, instead of riding forward out of the still stand, the wheel will be too much in front, and you’ll fall off the back after the stand still, hopefully onto your feet. (I suggest practicing all this on dirt; falling and sliding is less traumatic than falling and sticking to the pavement.) Now for the hard part: ride into the still stand, with the wheel out in front, freeze, then pedal backwards, half a revolution, to keep from falling off the back. Then, use the forward-riding skills you already possess to ride forward out of it.
The above method was a breakthrough, for me, and this is after I spent a lot of time practicing idling holding onto a fence. Learning “one” really good idle was, for me, more valuable than practicing 5,000 idles, assisted, at the wall.
Just my two-cents…