I agree with you regarding the committed scientologist. Arguing with them is useless, perhaps worse than useless. From my experience, not only do they not listen to you, but they become very angry with you. And, as was with my case, if the two of you happen to be coworkers, then the entire work environment can be poisoned.
But what I think should give us all consolation, is that at some point in their scientology careers, something happens that causes the scientologist to begin doubting. From my limited communication with those who have been on the higher levels- OTIII and beyond- I can't tell if it is the contradictions in Hubbardian reality and true reality that leads to OT withdrawals from the sect, or more mundane, day-to-day experiences with scientology, such as feeling that Hubbard's tech is not being faithfully followed.
For example, on a different thread, I asked an OT VII who goes by the name of Charles if he ever wondered why he wasn't starting to disappear ala Casper the ghost as he underwent more and more auditing. According to Stacy Young, the president of Lisatrust.com, Hubbard taught that everything in the physical universe was composed of BTs, including the human body. Therefore, one would think that the more auditing one has, the less "there" he would be. And for those who have had all of their BTs removed, they should theoretically not even be in the MEST universe at all, but just be a thought, theta, or whatever. Charles seemed to shy away from this question, not answering it directly. He simply explained that he was able to remove "charge" and apparently this counted as part of physical universe. On the other hand, several OT VIIs recently "blew" scientology because they perceived that security checks were not following Hubbard's directives.
And that leads up to the problem I have when confronting the committed scientologist. Personally, even though they aren't going to listen and will throw a fit when you start speaking to them about the real scientology; the Marcabian confederacy, Xenu, and all the other nonsense, I wonder if by saying these things to them, you aren't at least planting a seed of doubt in their subconscience. And then, maybe a month from then, a year or even twenty years from then, when the stress of scientological life gets to them, that seed of doubt from their subconscience will start worming its way to the front and they'll go over the top and back to the real world. I don't know..., what do others think; is it a good idea to confront the committed scientologist or not?
And as for the uncommitted scientologist, the one who hasn't been brainwashed or whatever it is that the scientologists do so well to their new initiates, I feel that it's paramount to get the truth about scientology in front of them. Just simply tell them what it is all about. Once they have a true understanding of scientology, before their brains have been snapped off, they are going to avoid scientology at all costs. I know that from personal experience.