Maintaining notes allows you to constantly append/edit these thoughts, essentially treating the notes as scratchpads. It allows you to leave your thoughts in an unfinished state, without exposing them to public scrutiny till you’re done with them.
On the other hand, there is an element of finality to pushing the publish button. The extra effort it takes to dot the ‘i’s and cross the ‘t’s hones your thought process. There is also something to be said about maintaining a web presence in today’s economy. It also encourages feedback in the form of comments, allowing you to have a conversation around the topic. From the point of view of clarifying my thoughts to myself, the only thing better than writing them up is having a conversation with someone about them.
The social networks today (Facebook, Google+) are certainly biased towards brevity. They are full of short snippets of random thoughts, often with no original intellectual contribution from the author.
I just noticed that WordPress allows you to “publish” a blog with Private visibility. I guess that could be the best of both worlds, allowing a smooth transition from an incoherent personal note to a publicly visible post.