The thing is, the book (PHB II) will come out in march, and it'll probably go to print sooner than that (say, mid february, I honestly have no idea).
We can't really playtest something if it's not going to be changed between now and the time they publish the book.
The barbarian and the bard already got some love, there's still 6 classes to go. Now Druid, Invoker, and Mercykiller (whatever that means).
We're missing another 3 (or 4 if Avenger =/= Mercykiller), though, they might as well not give us the other ones until the book arrives.
But in order for them to tell us what are the changes they're doing to the class based on the feedback from us, they (and us) need the time to do so.
That being said now, do you really think there's going to be any more of those articles? I'd say either they'll throw up all the last 3 ones for january, or possible a single one, or most likely none. So I take it much like some sort of a christmas present.
Although I do agree that it could get a bit overwhelming 3 new classes to playtest.