[evanny]

steven. yes. native speakers can and do speak incorrectly. if they didn't there wouldn't be a need for a class in the native language.
when a native speaks he knows that the other native or even non native partners will understand him without realizing how grammatically incorrect he might be.. i'm talking about the "correct" way.

my friend is studying at Birmingham university for the 3rd year. plenty of native speakers give her their essays for correction. she even was shocked about how many mistakes in expressions and grammar (specially mixing up times) they made.

i'll stress it again. i am talking about proper english and which one is better suited to teach it. native or non-native.
P.S this isn't aimed just at Japan but also in language teaching in general - spanish, french etc.