I've explained my position and I accept its downsides (as it happens I'm in a Lab/LD marginal).
That said, the framing in my previous post was unduly negative.
The positive framing is that the only way to influence Labour to be more progressive is to withold one's vote. For most disaffected Labour voters, that means voting Green, in the hope that the loss of potentially key voters in some seats pulls Labour to the left, just as UKIP pulled the Tories to the far-right.
It might be a slim hope but, from where I'm sitting, so is the idea that some of the most awful people in politics will develop a social conscience, and a sense of basic human decency, if they gain power.