Utilitarianism and consequentialism. It used to be a given amongst socialists, but not anymore I guess. You are obligated to perform the action that ensures the greatest happiness for the greatest number of people. Voting for the lesser evil is exactly that. An imperfect action, but a necessary one.

Honestly, socialists should always vote unless they have a specific strategy in not voting. Maybe they're boycotting a rigged election to diminish its ability to justify the government. Maybe it's because a specific deal with a party was broken and they're following through on a promise. With those examples, not voting is combined with a specific action to weaken the government. There's careful thought and the efftect of the boycott is maximized. This isn't the case right now. The only proposed strategy is accelerationism.