Private groups allow you to have discussions you can't have on the mains, and they are often moderated in a way that members find more agreeable (i.e. certain things that aren't allowed here are allowed there, but things that are allowed here aren't allowed there). It's easier to keep out trolls who make dozens of alts every time they're banned, and it's easier to forge connections with members because of the smaller community size.
I mean, I like the mains well enough. I like venturing out and seeing opinions and perspectives that private groups generally weed out over time. But were I forced to choose one, I'd choose private groups any day. Curious to see which choice others would make.
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What reason is there to post in private groups? To communicate with other beings, I'd think. I see private groups as a retreat - you can escape opinions you don't like, you can escape the uneducated, and sometimes the bigotry. However, you lose far more than you gain by posting in a private group. The audience in a private group is limited, but worse still, it's filtered - the people you converse with have been selected to converse with you on a set of principles, ones you have in common. Groups tend to become circle-jerks at a point in time. Not only can you converse with the same people on the mains as you do in the private groups, but you get to talk to many more too! You get the opportunity to educate others, and at the same time you get the opportunity to learn from others as well. Isolation isn't a good thing. Edit: perhaps the Flood wouldn't be so "bad" if all the "good" posters didn't retreat to groups. [Edited on 01.02.2013 6:55 AM PST]