Speech does not necessarily reflect intelligence. When I was at Cal State Fullerton, one of my business professors asked the class a question, "If you had a salesperson that seemed to get along with your customers but he had a strong southern accent would you keep him?" The majority of the class said no that they could not have some redneck represent them. Most equated a Southern Accent to being ignorant.
I imagine it would depend on the situation. If you were selling computer software in New York, a strong Southern accent wouldn't be an asset. Selling used cars in Mississippi would make you rich if you had one. On the other hand, a Brooklyn or Boston accent wouldn't help in the deep South.
When I lived in Knoxville the local district attorney would drag out his accent when he was addressing the courtroom (when they showed clips of trials on the news). Despite the fact he likely attended UT law school, he was probably a pretty bright guy.
To me, an accent doesn't matter. Strong command of the English language is a mark of an educated person to me. I cringe when I hear a thick Southern accent using bad grammar.