And you have never experienced that there are correlations between new features, changes, events and even bug fixes? A version that worked quite fine can show major issues after minor changes. Quite often we even noticed new issues when the versions were updated for the last time on Mondays/Tuesdays before their live release on Wednesday.
In my opinion and according to my experience (in the game and in my job) it is important and necessary to set a "design freeze" date and to test the patch in the actual state, as it will be released to the customer (i.e. to the live worlds). Since now the version on Beta is (or can be) subject to daily changes, you can never test excactly the software status that will be released to the live worlds - there will always be something still missing, or something else already addded.
You don´t need Beta testers for the testing of single new features - there are other ways to make sure that they work. You need Beta testers to find the hidden bugs and correlations you can only detect while playing - and only while playing with all the new contents and changes and fixes already in place.