If he teaches error, it often goes unchallenged, because no one would interrupt the sermon. People sit in a pew week after week for 20 years listening to the same person. Just think how much more they could learn if they heard from a variety of people. Preachers tend to have pet doctrines. This is natural.
All saints have their own particular spiritual interests.
So now we end up with churches where there are a few focus, for example, faith, making a lot of money, healing, and speaking in tongues. The result is unbalanced believers. We need a balance of teaching and exhortation from different body members in order to be balanced. If we consider these problems we can understand why the New Testament does not teach us to have one pastor in each church preaching all the sermons, but rather a plurality of elders and meetings open for all so led to share their gifts. Maybe if the church format allowed for a 'multitude of counselors' we'd see less false doctrine developing in Charismatic circles.