The problem is that there often seems to be an unspoken assumption that every interpretation is created equal, that the insights of each member are equally valid. This just isn't so.
Every competent group leader, one who has put considerable time and effort into the study of the topic or passage at hand, knows his insights are more valuable and edifying than the "insights" of the (often biblically illiterate) group members.
As a participant in such groups, I am often put in a very uncomfortable position where I must (strongly and publicly) disagree with a dear brother who, regardless of his sincerity and godly character, simply does not have a clear grasp on the principles of proper biblical interpretation and/or the teachings of orthodox Christian theology.
There is something wrong with any format where the interpretation of a sincere but uninformed layman is placed on equal footing with that of a called and gifted (and in some cases, seminary-trained) Bible-teacher.
And I'm not the only one who thinks so. In an article called "The Hidden Assumptions of Small Group Bible Study," T. David Gordon points out five problematic assumptions behind many (probably most) small group Bible studies.
1. Participation is as important as precision.
2. Every interpretation or insight has some value.
3. The Holy Spirit does not give differing abilities.
4. The Bible can be interpreted well without special aids.
5. The Bible does not interpret itself.
In conclusion, Gordon says:
The Bible teaches that the abilities to interpret and teach are differentially distributed; the small group (as a medium) teaches (or implies) that the abilities to interpret and teach are equally shared. The goal of Protestant biblical interpretation is truth; the goal of the small group biblical interpretation is participation. Those who participate in small group Bible study should be very aware of the limitations of such an activity. There will be many gains in the area of mutual encouragement and social development, but few gains in the area of apprehending properly the biblical revelation.Here's the link.