Yes I know all of that. I just don't understand why they put so much effort in the non public preview.
Because it's the final version.
Look, when Windows 8 was finished, MS sent the OS to OEMs, and began prepping their servers and retail channels for release. They also continued making tweaks and fixes, which they release on "Patch Tuesdays", or faster if needed. It took about 2 months before they made the announcement after 8 went RTM, mostly so ads could be done, hardware configured and built, and CDs stamped, and during that time updates were made. Stuff like this is why you buy a brand new computer and immediately have updates.
When Windows PHONE 8.1 was finished, typically MS sends the OS out to the OEMs so they can make their firmware changes, but then they have to wait for the carriers to finish their testing before it actually goes live (the Carriers are the ones who send out the updates). MS doesn't control when we see the OS, or even if we WILL see the OS, the carriers do.
The Dev Preview shifts control of the OS update from the carriers only, to MS. So when WP8.1 was finished, they pushed it to OEMs AND released it through the Developer Preview. From there, they went back to refining and tweaking like they always have done. Considering we don't have the middle man (the carriers) in the way right now, they can accelerate their update cycle, so when they finish an update, they can push it out to us immediately. Once the carriers push it out, that changes. It's difficult to push an update for phones because they have to rely on the carriers, so that's why we see GDR updates.
There will likely be a version bump between what we have now (or what we have after the last update they squeeze in before carrier release) and what the carrier pushes because of things they need/wanted, plus if MS knows the carrier release is imminent, it would just be a waste of time to push the update through the dev preview channel.
There is a gross misunderstanding of what BETA actually means. The Dev Preview is not beta.