Some things need to be tested in the public:
In the earlier thread, "Today was a successful stress test!", the answer is right in the title - they were stress testing how the servers would run when thousands of users were trying to join them. How would they handle? Sometimes, other unexpected things can come up - maybe everything was tested individually, then when it all came together, there were conflicts? Maybe the combination of the stress and something else caused it?
It was time for it to be released to the public, even though there were numerous bugs making it semi-unplayable. The only way some of these bugs could be found and crushed in the first place was to release it to the public.