There's a balance to be struck here and what Dancer doesn't seem to take into account, is the fact that people are spending over £100k on these schools, so feel it is perfectly reasonable to have some time to think.
Schools recognise this too. As businesses providing a service, they spend a lot of money and effort trying to attract customers. The glossy brochures, impressive open days, lovely catering at all working open mornings, goody bags, opportunities to look again, scholarships etc are signs of a school trying to woo their customers. And yes, some are very popular with huge numbers taking the exams, yet they cannot be complacent and ignore the wishes of those customers, because each child can only attend one school and ensuring the cleverest accept is key to continuing to perform well and attract more students in future.
I suspect City have made a mistake this year. They won't have imagined the list would be full before Offer Holders Open Day. Their attempt to regain a bit of control in the market where parents can hold numerous offers and they can't be certain about who will accept, has resulted in a full school (assuming they all actually stick with it and don't pull out for a better offer) but probably quite a lot of poor publicity which might have a negative impact next year.
I would think that next year they might over offer by less again. Of course we don't know how many people find themselves with an exploded offer this year or how many of them would have had it as their top choice in the end. Even if there were only a handful who have lost the school they really wanted, all of those who have had an offer explode will feel disgruntled and that news gets about - not the publicity the school will want. Perhaps they will abandon the policy for next year,mor maybe not and perhaps other schools might adopt it.
However the more parents can express their dislike of this approach, both at City and at other schools too, the more schools who do need to be mindful of their customers, might feel reluctant to use this approach.