Importance of Proper Speech?

In her article on page 59, Delpit describes an interaction between a student and a teacher where the teacher constantly interrupts the student as they read.  Delpit argues taht this does harm to the student becuase they should be able to learn in their own dialect.  While I agree with this to a pont– we all learn better when we are in a comfortable environment, I also see a problem.  Shouldn’t students be taught proper speech in schools? If students are constantly taught in their own dialect, and are not corrected when they read aloud, will they ever learn the proper way os speech? If these students are to succeed in a career sense, shouldn’t they be able to be able to speak proper English? So essentailly my question is this: is it possible to go to far in understanding the student’s personal background and taking everything in context of their personal dialect that we lose sight of proper English speaking that is necessary to be polished in the workforce?