Although I have to admit that it is easier for me to promote free software if I do not mention that I'm a socialist/communist.Well, that's just it. I think the movement would succeed a lot more if we focused on the movement itself, and not tying it into other dogmas that we may hold.
This is a capitalist society at heart. If I present myself as a businessman, and promote free software on the basis of what it can do for my business, I will be much more well received than if I present it as a basis for a pro-communist or pro-socialist argument.
People will already be shocked by the notion of an alternative model to proprietary software, and that shock must be overcome for them to accept your argument; why saddle it with even more shock by tying another sociopolitical agenda to it?