I don't buy the argument that the spoiler effect is the fault of the new party or that it means people shouldn't try to start a new party, but the spoiler effect is real. It's the reason FPTP is so shit. And because it's real, many succdems do think a new party would be bad (again, I don't think that). I expected Corbyn to be one of those succdems, but I guess I might be wrong, which is good.