The problem I have with this argument is that some people, that call themselves leftist aren't actually. They are just pro red-facism. And even if we just went along with them, they would eventually find a way to backstab us. Lennin killed all the anarchists.

I am really against working with these people, because you can't trust them to have basic principles. I am not saying, that we should actively fight them, as long as they are useful in fighting facism. But we should still critique them actively and not let them invade genuinely leftist spaces. Not only because they are not to be trusted, but also because they could scare of other reasonable people and / or make them think that leftism is when you like the Soviet Union.