Before I start I want to affirm that churches should be attractive to those who come. They should be welcoming, easy to assimilate into, with excellent preaching and music (the best one is capable of). If we do not have a welcoming culture we are hurting ourselves and not representing Christ well.
With that said lets consider the downside of building a church that focuses exclusively on being highly attractional in order to grow. It is very simple.
First, the attractional model can create a consumer mentality among those it attracts. It cultivates the notion that church is about me (it isn't) and my needs (only part of the equation).
We have all watched attractional churches suck people from neighboring churches because it's programming and preaching were "cooler" and better than their church. This generates church growth in terms of numbers but is often simply the rearranging of seats by believers. It is not the kind of church growth the New Testament envisions.
Then there is this: When the next cool church comes along, the same people who came to your church move along. They came as a consumer and they leave as a consumer. And consumer Christians are not generally mature Christians because maturity requires something far different than consumption. It requires engagement! Putting one's faith and gifts to work and living in community with fellow pilgrims.
Without engagement there is no ownership and the back door of the church is as wide as the front door. If you want to close the back door there is something else needed: a high level of congregational engagement.
Engagement encompasses three areas: Engagement with God, engagement with people and engagement with service to others. The first has a lot to do with public services whether on line or in person. The goal is not to entertain but to help people connect to their Lord and savior.,
Engagement with others is equally important as the weekend services because it is in community that we often put what we have learned to work. It is where we receive encouragement and encourage. Engagement through the use of our gifts in service to others helps us grow and furthers the work of Jesus.
The New Testament talks about engagement at every turn. It is often the missing element in the western church where we put so much attention in being attractional that we don't grow disciples who are truly engaged.