The Wii U comes with a charging cradle too, but also enables players to replace the battery as its life dwindles (unlike the batteries on my now dead original PS3 Sixaxis controller) which is an excellent development.
And really, there wouldn't have been much of a downside if Acer went with that form factor instead: you'd get the cradle and keyboard in one piece, along with a built-in battery and a smattering of ports.
For one, it's a battery hog (that, or the battery's too small), lasting about three hours in my tests before going back to the cradle or having to run plugged in (the power cable's reasonably long at eight feet).