First, never isolate a finger. They are interconnected and isolation causes strain. I know, most of us are taught to isolate fingers and many silly exercise books condone it. Well, it is wrong. Believe me or don't. In forty years you'll think back and think "Malcolm was right." but by then it will be too late if it is not already.
Technique comes from moving properly, not moving more or from some exercise. If you can't do something it is either because you have improper movements or not the correct ones. It is possible to have both. More practice can't fix that, only proper practice can.
If you walk knock kneed for a few hours and your knees begin to hurt, walking knock kneed more isn't going to make things better, stop walking knock kneed and walking properly will. Likewise, practice doesn't make perfect, perfect practice does. What you should really do is find a better teacher who knows physics, anatomy and ergonomics. Piano playing is 99% in the arm, not the fingers. The fingers are a conduit of the arm. Like casting a fishing pole, you cast from the feet, knees, hips, abs, shoulders, elbow, wrist then fingers with copious amounts of contrary motions. Every motion must have an equal and opposite motion. It is the law. The fingers are important but power and control comes from all the other fulcrums and contrary motions combined with the laws of physics and your anatomy. If you poke someone's eye out of course your finger does the poking but it is initiated from the shoulder and elbow. Piano playing is the same but we also use gravity and arm weight combined with rotations, in, out, up, down, shifts and tilts. The arm places the fingers. Like playing hopscotch, the feet don't jump from square to square, the calves, quads, knees, hips and shoulders contribute to unweighing your body and they all place the feet where they need to be. The feet don't drag the body. Then fingers don't drag the arm. The arm places the fingers.
As far as it being a waste of time, if you never play anything like that in real repertoire, is there a need to be able to do it? For example, if you live in the city where the speed limit is 30 mph, why own a sports car that can go 130? All the technique you need you can get from your repertoire. I am an organist and rarely play stride or octaves so I don't have a need to practice them. However, once you learn to move properly, most everything comes easily and one just needs to figure out the proper movement.
Tension is bad. It means you are doing something wrong. It must be corrected because further improper practice augments and amplifies the strain and also hard wires the improper movement into the brain or "muscle memory" where it will haunt you forever. Tension is caused by muscular co-contractions or dual muscular pulls. Each muscle pulls one bone in one direction and we have multiple muscles which can pull on one bone each in their own directions. When you activate two muscles at the same time you pull the bone in two directions simultaneously and that strains one of the tendons connected to the bone and muscle. It is a tug of war and one of them will lose. This is the major cause of median nerve entrapment (carpal tunnel syndrome) because we strain our long flexor tendons which run through the carpal tunnel and the strain results in inflamed tendons which then press on the median nerve. The cure isn't drugs, splints, rest, more practice, injections or surgery, it is learning to move properly so the inflammation can go down. It can never go down if one continues doing what is wrong, no matter how much rest, drugs or surgery you have.
Here is a simple example. With all 5 fingers together, slowly wave bye bye. It should be effortless because all 5 fingers are moving in the same direction at the same time. Some call this "tapping." Now abduct your fingers (spread them out) and wave bye bye. Feel the strain? It is because you are using both your flexors/extensors and abductors at the same time. Hold all 5 fingers extended and wave bye bye with just one finger: Strain? Those pulls not only damage our tendons but they also hinder technique because you are trying to play in one direction and another muscle is pulling you into an opposite direction resulting in mistakes, uneven playing, tension or bad tone.
Find a better teacher. Your current one might be wonderful but what they don't know can hurt you or, sentence you to a career of mediocrity.