Roberto von ArchimboldiVoting in national elections is pointless. This is true irrespective of the details of the electoral system. Your vote is vanishingly unimportant.
It does not follow that electoral politics should be ignored. It does mean that if you do think it matters who is in Parliament and you want change, you have to organise. Building a movement is not pointless. It is how a group changes what is possible. Look at what the DSA achieved in New York. If you don't want change, you don't need to build a movement. You can just encourage people to vote.
Do note that demanding that people shut up and vote to keep out the bad guys is to maintain the status quo. Your movement for change might decide that it is not in a position to win and so lend its support to a status quo candidate, but that is not the same thing as demanding unity. You don't shut-up.