Yeah,
The reason is basically what Kushan has alluded to. Functions frequently get broken in the beta releases (when new features are added). People aren't going to be too happy about that.
With the mk3 2006, menu 0.4 actually comes pre-flashed on the neo-key. A beta version, marked as such, is provided separately. Now I don't know the exact reason why it was included, as the mk3 patching routine is completely broken (maybe the purpose was to give a preview of what's going to be available in the offical release?)
The testers actually do their jobs quickly, by reporting any significant issues (we want to see a better product also since we're users ourselves)
Anyways, I hope .6 turns out well since everybody has been waiting a while...