Perhaps, on the other hand tennis is hard on the body and perhaps it is not so unwise to let the youth under 10 play with softer balls.
There will always be parents who think their son is going to be the next Federer if only they can start with yellow balls when they are 6 but that is not a very realistic prognosis.
The advantages I see with orange and green balls for young players is that:
1. The rallies are longer, so more practice.
2. The balls are slower, so more opportunity to work on technique.
2. Heavy topspin highballs can actually be returned instead of bumping over the head.
3. Less strain on the body.
I have to disagree with much of this. Talk to parents of kids who play lots of green or orange ball. The kids have to swing like crazy to get the balls over. Many a parent and kid has complained to me about arm pain when using those balls.
Try it for yourself. Go play with the orange balls for an extended time, really hit them, do it for a few days. You will find you have to swing so hard to generate pace. Tell me after a week how your arm feels.
Technique can be taught with yellow balls. My students by age 7-8-9 are winning in the 12s with great technique. A good coach teaches good technique with any ball, a bad coach does not no matter the balls used.
The orange and green balls are a false sense of things. We have these kids go from orange and green to yellow and get smoked. They try to come to the net and get passed easily by a trained yellow ball kid. Once they realize real tennis is hot sun, 2 hours, moving the feet to position for yellow balls, many of them quit tennis.
As far as more practice, not really. A long rally with soft balls is not practice. the only way to groove strokes is rep after rep after rep with a coach or ball machine to groove the strokes. Hand fed drills for movement. Rarely do kids improve a ton by just rallying, no matter what ball they use.
It is what it is...every section has kids who spend hours per week drilling. They groove the strokes. Groove the footwork. These kids dominate and finish 1-2-3-4-5 in every tournament. The other kids finish past that, no trophies, quit tennis after a while. No easy way around it, and using slow balls is not going to change this reality.
My 7-8 year olds students never have heavy topspin go over their heads because they have been trained to not wind up, put the racquet behind the ball, and take them on the rise.
Do not underestimate kids. A good coach can produce kids in 12-18 months that by age 7-8-9 can play great tennis with yellow balls on a full court.