This is one of those subjects on which I find it difficult to come down on one side or the other.
I admit to liking to see children dressed smartly in school uniforms and I can see that, in some cases, it can create a feeling of belonging, cohesion and pride in your school (though, apparently, this isn't necessarily the outcome). I also feel that having a basic uniform to some degree prevents the sort of fashion one upmanship that might arise if children could turn up in whatever they liked.
But I also think that schools should be about gaining knowledge and learning new skills, and thinking about what makes a decent society and how each individual - whatever their abilities - can contribute to making it so. I think the rigid application of rules re uniforms suggests that artificial and irrelevant goals are being set that have very little to do with true education and more to do with conditioning children to accepting "rules". Some might say it's important for children to obey authority, others might think it is more important for them to develop self-discipline and a sense of responsibility.
I think the headmaster who sent home a girl because her shoes, though flat and black, were suede and didn't conform to what he thought was appropriate, was being overly picky.
Apparently most European state schools, including Germany, do not have uniforms and it doesn't seem to have affected their education system.