I think that a polo shirt is more comfortable than a collar and tie, and looks smart enough too. I wouldn't dress a small child up in collar and tie except for a really special occasion, and can't see why it is neccessary for primary school children.
Primary school uniform needs to be practical, easily washed, and tough, so that it doesn't get in the way of whatever the children are doing, and they don't have to be careful of it. I'd rather have my kids in a sweatshirt or fleece that they don't have to worry about much, than in a blazer that will probably have cost far more than the sweatshirt/fleece, so they'll have to take extra care of it (and we all know how careful primary school children are of their clothes).
If a sweatshirt gets dirty, you can fling it in the wash, tumble it, and it won't even need ironing - and you will probably have been able to afford at least one spare - whereas a blazer will not be anywhere near as easy to clean when they've tipped custard or paint down it, or covered it in dirt using it as a football goal. Plus, as it will probably have cost more than the sweatshirt, it's unlikely that many parents will have more than one, so that will be less convenient for them.
Also, polo shirts can be washed, tumbled and folded - and even if they do need ironing, they are much easier to iron than a shirt. You may be lucky and may be able to wash and dry a school shirt so it doesn't need ironing, but I have never managed that. Ds1 has finished school now, and is off to uni in the autumn, but for the past few years, I have had three boys at senior school, needing a clean white shirt each day, and dh working in an office, ditto - and that is 20 shirts a week to wash and iron! I would not have been happy if I had had to do that all the way through their primary school years as well.