The devs cited feedback from current beta testers in making this call, and addressed other concerns that players have expressed. The team said that it is working on lag issues, a better match making system, new player tutorials, and general balance tweaks. Players who have paid for a premium account will also have the option to decide when they want to start using the benefits from the bonus.
There is no word as to when the open beta for MWO will commence.
[Thanks to John for the tip!]