I think that it is a matter of quantity changing into quality. If someone's spelling doesn't follow any recognized standards and conventions, their writing becomes incomprehensible to the general public irrespective of whether the thoughts they are trying to convey are deep and logical or total nonsense. I do believe that it is necessary to instill a sense of the importance of spelling in students, that it does have a function and that it's not just some form of tyranny by the teacher. But it is even more important to encourage people to express themselves regardless of their competence, especially people with learning disabilities such as dyslexia or aphasia and learners of English as a 2nd language such as... myself! :)