Remember the "Secret Friday" updates? What ever happened to them? Personally, I see the weekly pre-releases as a sort of replacement for the Secret Friday updates of the past. Why? In some ways, the pre-releases contain secrets, just as the Secret Friday updates did. Really, the main difference now just seems to be they come in the form of "unpushed"/manually installed pre-release updates.
Snapshots aren't supposed to be stable and mainly serve as play testing so bugs can be reported without them ever being part of the game. Secret Update Fridays were the same as snapshots in the sense that they were buggy and unstable, but were unfortunately pushed as actual updates.
It was ok to automatically push out experimental updates in Alpha, but now that the game is past its "full release", and there are 5 times as many players to complain about bugs, the current method is the way to go.
We still get little secrets in the form of new undocumented features, and I think the voluntary-download method is spot on.