Of course if you use a non-stock messaging app, like Handcent for instance, you may not want to switch to Textsecure for other features/reasons.
Text secure is great, though. Really, really great.
Unfortunately, there are no explanations on webpage and I'm not currently into the mood of reading source code to figure things out.
Maybe outgoing (unencrypted) SMS sent through TextSecure app are tagged as "hey, we can upgrade this to secure comms". Then all you have to do is use the app. But I really don't know.
The next-gen data-based TextSecure protocol (which is currently deployed in CyanogenMod's WhisperPush implementation of TextSecure, but not yet in the Play Store app) uploads a hashed list of the user's contacts to the server for contact discovery.