Jiva Gosvami explains that if one cannot respect devotees, then how can he preach? If he cannot respect that somebody is actually superior to him and somebody has some qualities that can help him make progress, and if he sees that way in the association of the devotees, then how can he respect the non-devotees? Then how can he preach? Therefore, Jiva Gosvami is explaining that such persons are useless for preaching.
Taking Care of Devotees, Pg 97, 3rd Paragraph