Being opposed to religion is not the same as trying to prevent people from practising religion. We're not interested in setting up an enlightened dictatorship. Socialists don't want to police peoples thoughts - rather, we seek to change peoples ideas by engaging with them. It would be stupid to think that, in a society based on the endless oppression and coarsening of the worker masses, religious prejudices could be dispelled by purely propaganda methods. To put it another way, rational argument alone will not convince people to abandon religion because religious conviction is not primarily arrived at through a rational process (people don't generally become religious because they've sat down and thought through the issues but due to indoctrination, spiritual experiences, etc.) We understand that ideas and consciousness come from our interaction with the real world, not from some outside force or god. From our perspective, people 's ideas change as a result of the struggles they engage in. If that were not the case there would be little point in our engaging in propaganda or activism - we simply would be unable to convince the huge number of people in the world of our ideas.
Likewise it is obvious the workers of the world won't just wake up one fine morning, decide religion is bollox and go out and wreck the avenues where the wealthy live. So it is certainly worth engaging with these sorts of beliefs through propaganda. Of course ideas are always produced by the material and social conditions of the day. We'd be very poor materialists if we failed to spot that! But even so, ideas themselves do carry weight and are worth engaging with, or opposing, both individually and as part of an overall belief system that is equally a product of the times. We should strive to educate people, not ignore their ignorance.