Even in the context of the group study, they will exhibit far less attention and participation and show far less application. Everyone has different strengths, weaknesses and needs. You can't deal with all that variety in a group situation. Frank thinks about nothing but his neighbor's wife all day. Jed's thinking about dropping the study next week because he can't resolve evolution and creation.
Individual needs require individual attention.
It's one of Satan's oldest tactics - separate the individual sheep from the flock. Break off all ties of spiritual accountability and encouragement, pour on the pressure, and then watch him self-destruct. People don't bail out of the Christian life because they didn't catch the sequence of Paul's missionary journeys, or because they only made it through Book Three of an eight-book Bible study series. They bail out because life got a little too rough, and when they needed someone to lean on for a while, no one was there. Every one of us has a hurdle or two that's keeping us from going a little farther down that road to maturity - and usually the tallest ones are the private, personal types that we'd never feel comfortable talking about in a group setting.