You've described my MO as of late pretty succinctly here. I find that writing a comment, with a series of edits, helps me to understand how I personally feel about a particular topic. Once I've ironed out the creases of an idea in my mind, it's less important to me that other people hear it.
That's funny, I go exactly the opposite way. I write out a response, and if it's nice and clear I post it because it feels like an idea worth sharing. Better clarified the idea, the more I feel justified in sharing it. Every once in a while, I'll type out a long-winded response, realize 75 percent of the way through that it's either inane or just based on some huge fallacy I didn't see before typing, and then delete it. Sometimes those are my favorite responses, come to think of it, because they end up re-shaping some basic assumption I'd held for a long time. But yeah, the ones I discard are the ones I don't think are worth sharing.