The attitude towards "Third Partying" in Scientology is comparable towards the attitude towards child molesting in mainstream society. As a result, talking about other people in their absence just isn't done by Scientology adherents. It could cause war, according to Scientology scripture (see below).
In real society, however, talking about people in their absence is an entirely natural part of getting along. Not only is it educational and entertaining, but it is also how people keep any one person from exerting too much control over others. It is a natural social defense mechanism against undue influence.
As a result, the action of "third-partying" is an effective antidote against brainwashing. That is one primary reason third-partying is heavily frowned upon by cult practitioners of Scientology. A secondary reason is that, during a police investigation, it is against the religion of the the people who are supposed to provide the investigators with information to do so.
Sadly, Scientology does not use third partying for constructive purposes, but for destructive purposes, like war:
From http://www.scientologyhandbook.org/sh8_1.htm "It is very easy to see that two in conflict are fighting. They are very visible. What is harder to see or suspect is that a third party existed and actively promoted the quarrel. The usually unsuspected and 'reasonable' third party, the bystander who denies any part of it, is the one that brought the conflict into existence in the first place. The hidden third party, seeming at times to be a supporter of only one side, is to be found as the instigator. This is a useful law in many areas of life. It is the cause of war.' Using the above as a guiding principle, Scientology OSA operatives play the innocent bystanders and use third-partying destructively in an attempt to dehumanize Scientology opponents.
Another example of Scientology using an otherwise constructive item for destructive purposes is its use of the sauna to sap adherents' strength in the "purification" breakdown or "rundown".
Speaking out on behalf of constructive cults, Joe Cisar http://cisar.org/rfs0100.htm Overview of Scientology in Germany: http://cisar.org/overview.htm Why would Gottfried Helnwein, one of the world's leading Scientologists, lie? See http://members.tripod.com/German_Scn_News/has00.htm