KFW, you certainly have a low opinion of karate / TKD roundhouse kicks...
The discussion is whether the roundhouse kick with the ball is worthwhile for your training time or not. It wont' work well if you
- have shoes on.
If you can kick with the top of the foot and with the ball of the foot, then you can do what you need to with foot wear. Just because you are wearing shoes, doesn't mean you always want to slap the other guy with the laces. If you can articulate the shoe, you could still use the ball. You could also be able to adjust the angle of your foot, to strike with the sole of the shoe. If you are wearing steel toed work boots, you won't be able to use the ball... but the steel toe will leave a mark. Its about control, and options.
- fully rotate your body.
If you mean fully rotating your body in a circle, MT style, then you are not using the top of the foot, or ball of the foot... you are using the shin. If you are talking about a karate / TKD style kick, you would have the same full body integration, regardless of which surface you strike with.
- use low roundhouse kick (to hurt your opponent's leg).
Why? There are targets on the leg, that will do damage... and they are easier to hit with the ball of the foot than the higher up targets. Thats why beginners are able to start with low kicks. Any target you are aiming at with the shin, can be hit with the ball of the foot... and will do similar amounts of damage.
Does Karate low roundhouse kick (to hurt opponent's leg even exist?
Yep. So do low front snap kicks, to the opponents leg. You can target the ankle, the calf, the knee (I prefer using ball or shin on the knee) and both inner and outer thigh. You can also target the hips and groin, though I find the ball or heel of the foot better for attacking the hips.
The MT kick generates more power and hits harder. Its also slower, more telegraphed and results in an over rotation on a miss. The karate / TKD kicks are slightly less powerful, but are faster and don't over rotate you. Which kick does more damage? The one you can land in a given situation. The MT kick may be 100 times as strong, but if the other guy sees it coming, steps out of range, then takes your back... what good did it do? Learn both. Practice both. Learn the pros and cons of each. Learn when to use each.