Have a look here for an excellent article on 'why plant churches?' by Tim Keller.
General argument = Church Planting is the best way to be true to the great commission, to renew and mature the church at large and to reach new communities.
I can't help but agree.
All of which is not to deny the great need for energy and resources in existing churches. It is merely to recognise the fact that we should always be seeking to break new 'ground' with the gospel (new communities, new ethnic groups, new cultures and sub-cultures) and that means planting new churches.
I've also heard recently the concern expressed that 'church planting is just the latest fad' (just like 'seeker-friendly church' was all the rage a while ago and before that small groups and so on). This article gives good reasons for suggesting that it isn't (in terms of the basic God-given gospel mandate which amounts to a mandate to start new churches) and therefore that it certainly ought not to be (in terms of our own godly planning and appropriate gospel-strategising).
In recent decades evangelicals have been fascinated with the 'revival meeting', the evangelistic 'crusade' and the para-church organisation (none of which are necessarily bad of course); at various times all of these have been regarded as the correct strategy to spearhead mission activity and Christian renewal. This has often gone hand-in-hand with an impoverished doctrine of 'church' which has surely in turn aided the proliferation of an individualistic understanding of Christianity and helped drive a wedge between discipleship and evangelism.
Time to put 'church' back in The Church methinks.