I think that if you are going to be arguing against a philosophical position online, then typically it is in your self interest to read something by a philosopher who has defended that position.
The main reason is that it gives you a more detailed understanding of the position in question. In an online debate, your opponent might not have the patience to type out an entire philosophical theory in detail, or, more sinisterly, he might deliberately leave certain components of his argument implicit so that you have to work harder to figure out why his argument fails. It helps to be able to say “philosophers who defend your position almost always argue ABC, which is wrong because XYZ.” This creates a presumption that you have refuted your opponent’s position which your opponent will have to try to overcome, either by clarifying how he disagrees with the way philosophers usually defend his position or by refuting the response you have made.
Another reason is that it helps you know what to take seriously. If your opponent makes an argument that looks vulnerable to an easy rebuttal, is it really a weak argument or does he have something forceful to back it up with? If you have read the philosophers who defend your opponent’s position, then it’s easier to tell which of his arguments are strong and which are weak.