You are assuming that there is a question to legitimize. If you have someone convinced of the issue then there is no question. By engaging in discussion you are

You are assuming that there is a question to legitimize. If you have someone convinced of the issue then there is no question. By engaging in discussion you are simplying providing an alternative view point. You don't have to present yourself as opponent, you can present yourself as member of the ingroup with a slightly more moderate position that might also be more palatable to the public. There are many strategies. It is also important to realize that there are many passive readers of such discussions. I recall moot (rip) saying something along the lines that there is a 1:10 ratio of posters to lurkers. So by presenting a more moderate position you're also showing people who might not like the left that there are alternatives to the most radical voices. In the past, even in many brutal wars, belligerents generally recognized the need for parley. This is not even a war, it is a disagreement about social norms and laws. If villainize the other side to the extent where it would be immoral to even speak to them you're only fueling the polarization instead of creating a continuum. Look at europe. The extreme right exist too, but between the left and the right there are many different currents represented in parties, which lowers the activation potential for people gradually switch camps.