Maja - if child is struggling to grasp the rule when taught implicitly, they are likely to be the very children in the class who will struggle to grasp it explicitly. And what makes you think phonics is only being taught to five and six year olds? Let's talk about the process for seven and eight year olds, when teasing and humiliation is more of an issue - listening to a word being sounded out when you already know it can be intensely irritating and that needs to be acknowledged because a secure learning environment is paramount.
Maja and Cecily - I don't know if you are only familiar with small class sizes where one-to-one attention is the norm, but this is not the environment in which most children in which most children learn to read. Perhaps you could reflect on how much time your children have had sounding out words to you in a very safe and affirming space and then imagine their progress without your input.
Cecily - Your point about known vocabulary doesn't go far enough. There will always be words that children don't know and it's likely to be the less able children who don't know them. Also, it can be paralysing for children to know that they are supposed to know the words they are sounding out. Have you ever said "Come on, you know this?". I've heard it so often! Along with a titter from bored, more able children who are, quite understandably, assuming that it's easier to read a word you 'know' than a word you don't.
Aside from a TESOL diploma with some overseas experience of primary level teaching, I've also entered a school to intensively teach reading to (amongst others) a very gifted, autistic seven year old on a one-to-one basis over the course of an academic year. I hold an MA in the Written and Verbal Arts and have carried out multidisciplinary postgraduate research relating to children and their treatment of written stories. I've also volunteered extensively in a number of different schools, listening to children work with stories in real-time. But I'm not a professional primary school teacher and my observations come from the 'outside', if you like.