From Tim Keller’s Why Plant Churches [PDF]:
The vigorous, continual planting of new congregations is the single most crucial strategy for 1) the numerical growth of the Body of Christ in any city, and 2) the continual corporate renewal and revival of the existing churches in a city. Nothing else–not crusades, outreach programs, para-church ministries, growing mega-churches, congregational consulting, nor church renewal processes–will have the consistent impact of dynamic, extensive church planting. This is an eyebrow raising statement. But to those who have done any study at all, it is not even controversial.
Helpful article which responds to church planting objections and has some eye-opening stats such as:
Dozens of denominational studies have confirmed that the average new church gains most of its new members (60-80%) from the ranks of people who are not attending any worshipping body, while churches over 10-15 years of age gain 80-90% of new members by transfer from other congregations. This means that the average new congregation will bring 6-8 times more new people into the life of the Body of Christ than an older congregation of the same size.
The stats and story in Appendix A are noteworthy as well.