For me it all goes back to Krashen’s statement that true acquisition of a language happens on the road that meanders through the unconsious mind. Languages are just way too complicated to leave to the woefully inadequate devices of the conscious analytical mind. Bryce has put together, over on his site, a very nice little chart to show how acquisition and mere learning differ:
http://www.brycehedstrom.com/2011/a-bit-of-s-l-a-theory-110711