The size of the spacebar on a standard keyboard takes up the space of 6 keys; but people will push the spacebar in the same place. It's nicer for the thumb to be able to access more keys, and nicer to not need to rely on the pinky finger for so many keys.

I think most keyboards which give the thumb more keys also follow this trend of taking away the numpad, F-keys, arrows; often even the number-row as well. (Maybe just my bias, but I think most keyboards with more thumbkeys are symmetrical, too).

While there's a trade-off "moving hand to a greater number of keys" against "complexity, like layering".. I think the small keyboards make more efficient use of the hands compared to standard keyboards. -- My guess is that people who try designs with more thumb keys then think layering is pretty neat.