Ignorance is not all or nothing. Everyone knows something, even if it is very little. Yet even if it their knowedge is vast, no one knows everything. So we are all ignorant to some degree. When we are dissatisfied with our ignorance, we seek knowledge. A teacher is someone who has the knowledge we seek, and will share it.
Which is worse, a teacher without knowledge... or a student who denies his ignorance? Martial arts has both.