I have full knowledge that as I write this there will be people who disagree with me, including people I know who are planning to work in para-church ministries.
The Para-Church exists to meet needs of the community outside of the church walls. The idea is that people may not feel comfortable going to church, or that the church is not meeting felt needs well enough. These organizations may offer counseling, job training, food and shelter etc. that those around them truly need. This is all done in a Christian atmosphere but with no "Christian requirements" - the people receiving these services are not expected to go to church, read their Bible, grow closer to God etc. It is encouraged, but not pushed.
I think the only reason these organizations exist is because the church is not fulfilling it's responsibilites to the community at large. We are called to visit the sick and imprisoned, to clothe the naked, to feed the hungry, to give water to the thirsty. Historically, the church has done this very badly. During one of the famines in Ireland the people (most of which were pagan) were given the opportunity to receive food from the local church, if they would first go through confession and convert. The heart was right (at least I hope it was) - save their bodies and their souls - but we cannot turn God into a commodity, a bargaining chip. Come to church on Sunday and we'll help you find a job. Be part of this small group and we'll give you food. To do this is at best spiritual manipulation and at worst (and more likely) spiritual abuse.
Yes, we want people to be part of Christian community. Yes, we want to meet their needs. But the church should be able to perform both functions.
We see examples of this through Acts. Almost everything the early church did at the time added to the number of those who were being saved, and most of what they did would not fit in to what we call "evangelism". The met needs, they helped people, no strings attached. They loved, and that love brought the people they were loving on to God.
Didn't Jesus say they would know us by our love?
Not by our promotions, not by our preaching, not even by our relationship with Him, but by our love.
I think the ministry of parachurch organizations is good and necessary. But I think if the church fulfilled what it's function truly is that it would not be necessary.
Take a look at this church. They see a need, they meet it. The love, and the Spirit of God adds daily to the number of those being saved.