This is a very intersting topic, and one that has been discussed by cricket-minded folk in my area many times. Of course the ECB guidlines are pretty clear and I can understand the many reasons why they are enforced.
I think that juniors starting out in hardball cricket find it not only difficult and even off putting wearing lids, but they also feel clumsy and odd wearing pads, gloves, a box, etc. In my experience (one son 26 the other 14), but also of qualifying as a coach way back in 1996 and having coached and managed teams since (in the UK), is that it soon becomes second nature and they quickly get used to it. There is also the argument - and actually it's a strong one - that wearing a lid keeps ones head straighter and stiller, eyes more level, etc. And of course, should a youngster get hit on the head whilst not wearing a lid, this can - and has - put many a young player off and can develop a lack of confidence.
The costs... well, it depends - our club has decent kit for our juniors, so many do not have helmets themselves, although lots do. It can - as we all know - be an expensive sport (just shelled out for my 14 yr old a new B3...) and some people are in a better position than others, but most parents seem happy to provide kit especially for safety purposes, and whereas I agree - it is unlikley that an U11 bowler will get such pace and lift to scone someone, it also stands threfore that a kid of ten doesn't really need a £250 lid and just about anything will suffice.
It is a good habit to get into and as I say, they do so pretty quickly and it becomes second nature. Being of a certain vintage and still playing (although I did retire after the last game of the season only to come out of retirement that very same night.... once the ale had eased my aching bones), I have never worn one (although I wish I had earlier in the year as I did top edge one into my dish, fracturing my right cheekbone - although I did bowl 16 overs after doing so and played on the Sunday, after which I suggested to the wife I should perhaps go to A&E...). But that is really the first such injury I have had and the guy was sharp and I am old!
However, I feel it's both sensible and beneficial for junior players to wear lids and get used to wearing one as early as possible, so that it becomes second nature. And although I cannot speak for other countries about cost, etc., it doesn't seem to be restrictive in my experience and that clubs very often at junior level meet both the need and the cost of protective equipment.