It's considered beta likely because Apple isn't completely happy with its performance, and frankly I agree. I still frequently get "I can't do that right now". It used to be 50% of the time. Now it is maybe 10%. Regardless, unless I simply cannot use my hands I almost always just go to manually entering the task as I know THAT will work every time. My feeling is Apple will drop the beta tag with Siri in iOS 6.