can anyone enlighten me on why this is still being defended at great cost to the PM, the government and the wider public health message?
Because he's the best they've got. The whole shambles of a Johnson and cabinet was assembled by Cummings as a bunch incompetents who he's got dirt on so he can largely keep them under control (see synchronised social media messages from them over the weekend).
They're all frighteningly incompetent and venal. He's the only one with any brains and ability at all.
That he wastes it on being a eugenics-loving libertarian narcissist (what a collection of mental issues!) is neither here nor there. Without him, the whole hard-right cabinet falls over in a puddle of its own incompetence.
Either that or they really like him as a person.