The best reason there is, to have something to dream for. We see it in tons of our literature, look at how popular Star Trek is. It's not the crazy sci-fi stories that had people coming back, it was the world the audience wanted to live in. I wouldn't stomp on someone's dream like that. Especially when it is widely impossible, like who cares? It's their dream.

It's shortsighted to think that global communism is achievable within a human lifespan just as it is to assume that system won't be as vulnerable to corruption as current systems are. However that's beside the point, why advocate for something so widely impossible; because maybe it gives people hope that things may get better. If it was possible I'd definitely take more interest cause I don't think it'll work out how a lot of people who advocate for global communism think it will, but it's not possible.