i don't see how a beta would slow the whole process down. If they release only one level that they have done as a beta, i think that would be fine. it's not like they have to go out of their way to make beta levels, just throw some in that they have already designed, possibly the ones from E3 or Quakecon
assume there's a beta, instead of the few people they have testing, there are suddenly hundreds,
where there was 1 report of a glitch, now there are 50reports of one glitch.
fixing the glitch and then checking all those other reports is gonna take more time than it would have to just fixing the glitch and ditching the report while saying "thank you"