If our opponents are wrong and we're right, some people will eventually say "hey there's a real difference here, I'm going to support the folks who are right." That was a real appeal of the left to me and to other leftists I've discussed this with. You can look as deep as you want into leftism and still find that things check out, where looking into other ideologies even a little quickly brings out major contradictions.

It usually takes more effort to do good things, and do them right, than to do bad things.