No, allowing fringe groups to exist in politics does not afford them credibility in the public eye, because the public insists on the exercise of free speech. Most people think that these fringe groups are ridiculous and off-base. With freedom of speech, people believe that they can criticize these groups. Censoring speech would have a chilling effect on everyone.
No, allowing extremist groups does not make them legitimate in the public eye, because people know that they are not credible. People know now to take these groups seriously. People also know that they are on the fringe of society. Society knows that people have a freedom of speech, so they know not to take these groups seriously.