I couldn't agree more. Often I write out a large response to a comment, then feel no need to submit it once I've written it. It just helps clarify ideas I have. Almost deleted this one.
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.