I think everyone would agree that it is really important to ensure a child have depth of understanding. It is great that more emphasis is placed on that across the board in education. I think a small amount of explaining can be helpful. But, the issue arises where explanation is relied on too heavily as the extension activity for more able children. Sadly, it seems to be used too often as a way of keeping the able children occupied and 'extended'.
Practicing verbalizing skills is crucial. But it is a slightly different skill. Although there was one person who said they loved explaining on the board, and are now a teacher, for some children who are more shy that would be excruciating. Some kids can't find their words, or don't choose the right ones. I still can't define the meaning of words very well, even though I have an excellent vocabulary. Like Irvine's son, my understanding is intuitive, and I know how to use them, and I understand them when I read them, if someone gave me a group of words as a fixed choice I would know what would define them best, but my ability to free-recall other words to define words is not great. I have the same problem when explaining concepts to people (which I have to do in my job). I am just not good at giving the most precise explanation, even though I have a very profound understanding of what they are. The words I use to explain, are sometimes (not always) just not ideal for communicating to others. There will also be the children who possibly have very advanced reasoning skills, but are less good at verbal interaction. Again, encouraging them to do some explanation is a good thing. But, it does not solve the problem of extension and challenge to very able kids. As some teachers have said, used correctly, and as a small part of extension great, but not great for children who get too much. For most kids, it isn't going to inspire them to crave knowledge, understanding and to enhance their thinking. For many kids, it may be worth while, but it will be a laborious and frustrating exercise.
For these kids, who may already HAVE that depth of understanding already. The system assumes that for a certain stage, in a certain year, children don't have that. Unsurprisingly, it works on the core percentage of children in the middle, with an acknowledgement of adjustment and help that is needed for children at the struggling end. At some level, I do understand why the system places most importance on those children at the 'struggling' end of the table, because the impact of not achieving is most detrimental to them. However, it is a shame that in many cases, the state system cannot understand, let alone accommodate the other extreme of children. These kids need more than just explaining as an extension. I think that probably most teachers on here would agree, but clearly the experience of some parents is that their children simply aren't getting that.