Daniel Finkelstein reckons the Euston Manifesto is "a gigantic waste of time and energy." I think he's both right and wrong.
He's right, because the manifesto's signatories will not change the minds of the egomaniacs and childish pseudo-revolutionaries who support tyranny and bigotry.
He wrong, because there is something admirable about working in a futile cause.
A great example of this is Colonel Nicholson in Bridge on the River Kwai. He set about building the eponymous bridge even though he knew that doing so would help the enemy. He did so partly to symbolize what the British stood for:
We can teach these barbarians a lesson in Western methods and efficiency that will put them to shame. We'll show them what the British soldier is capable of doing...It's going to be a proper bridge.
Not everything is worth doing merely for extrinsic purposes. Some things we do to signal who we are. People give to charity even if they suspect the money's just a drop in the ocean. They protest, though they know they won't change the government's mind. They vote, though their vote won't make a difference. They even die for their country and their friends.
Instrumental rationality is not the only rationality. There's also, as Robert Nozick argued in his best book, symbolic rationality - the effort to show who we are.
But it's not just this distinction that Daniel's missing. He's also missing the distinction made by Alasdair MacIntyre, between the goods of effectiveness and those of excellence.
The Euston signatories, as I see it (but almost certainly not them), are not pursuing the goods of effectiveness, in the sense of aiming to win converts. Instead, they are pursuing the goods of excellence. They are trying to advance a particular practice, of expressing a left-liberal tradition. This is worth doing in itself, whatever the consequences.
Now, I write as a non-signatory to the Manifesto. But this is because I think the (futile) goods of excellence, of expressing a left-liberal tradition, are best pursued by ignoring village idiots, not by engaging with them.
But this is a second-order difference in this context. Where I agree with them is in thinking some things worth doing even if failure is assured.