This post is in response to Chris' Twitter Condom post.
I've been on the fence regarding public or protected tweets on Twitter.com. I've actually toggled that switch at on at least 3 different occasions, and then reverted back to Public maybe a day later.
Public tweets are visible to the world, indexed by Google, and make it easy to nanoblog (something that can be encapsulated in 140 characters or less, which might not otherwise be blogworthy). Protected tweets are private, and are visible only to those people whom you follow on Twitter. They aren't indexed by Google, and they're essentially part of a private conversation.
The problem is that the public/protected flag is a global setting for an account. Either all of your tweets are public or they're all protected. There is no middle ground, or ability to change this on a per-tweet basis.
So, why set an account to be "protected"? One of the things that strikes me about Twitter is how much it feels like a conversation in a pub. It's informal, loose, and sometimes a bit more unfiltered than would be otherwise advisable. A common type of tweet (of which I am probably more guilty than others) is the rant/vent. Bitching about meetings. Letting off some steam in little 140 character puffs. Probably not something that should be indexed by The Goog. To be really honest, probably the kind of thing that shouldn't be online in the first place, but that's probably another post.
If a twitter account is set to be "protected" then it becomes a bit more safe to open up a bit more (too much?). The pub conversation becomes more intimate and real.
Why set an account to be "public"? It makes it easy for people to follow you. Your social network/graph can grow without obstruction, and you will likely find new people who are doing things that interest you.
But, it's not as simple as it sounds. I currently follow 70 people. That's about the maximum I think I can follow. There are 318 people following me. It's just physically impossible for me to reciprocate. Am I missing out? Possibly. But anything important will trickle through various conversations and I'll see enough to make sense of it. And anything really important will likely exist outside of Twitter.
And I doubt anyone would really miss anything by being unable to follow my tweets. Sure, they'd miss out on some extremely witty banter, but anything important would show up in other conversations, and eventually outside of Twitter.
To me, twitter isn't a publishing platform. It's an informal hangout. If I want to publish anything, I'll put it on my blog, or as a comment on someone else's.