The troop I was in as a boy (254 Chattanooga, TN) strongly encouraged boys to have a full uniform shirt, trousers or shorts, baseball cap, and red and black neckerchief. I never owned the shorts, wearing trousers all the time. We always wore the uniform when traveling to or from a trip and every morning at summer camp (with one exception, explained below).
No scout was required to own a uniform, but if you wore one you wore the whole thing, except that we sometimes left out the neckerchies or allowed boys to wear a camp neckerchief instead of the troop one.
The uniform taught boys to take pride and put effort in to their appearance, without getting in to a materialistic idea of having in style clothing (many boys were wearing experienced uniforms. some even still had old style green uniforms).
We encouraged boys to learn to sew their own patches and let their pants out when they grew. So the uniform taught us life skills.
I also learned to iron by taking care of my uniform.
When we alone of all the troops at camp turned out for the morning flag raising in full uniform, other troops noticed! traditionally on the next to last day of camp, we would all turn out in beach wear and or bath robes (one boy wore a towel once) and the look on the other troops faces was always priceless.
In contrast to other troops that had more lax uniform standards such as shirts with "whatever" pants you wanted. Our troop was and apparently still is the largest in the council. We managed to produce at minimum, two eagle scouts a year.
I think when boys see scouts uniformed propperly, looking like their uniforms are a source of pride many of them are drawn to scouting. When we look like a bunch of kids who dess slopily and like we don't care, they don't want to become part of us.