That's one argument Himalaya and I agree that it's a valid one but, on balance, I still think these trips are helpful.
For one thing, from what I've experienced in brazil, it is not part of the local culture to do things for free. They're not malicious, uncaring people (anything but!) but they just don't think about it. Most charity work seems to be done by foreigners.
Yes, the organisation could have sent the money over but, realistically, how many 16-25 year olds (average age of of a volunteer I'm guessing), are going to send £2000 to an organisation to employ local people to do the work. That might be the ideal but it wouldn't happen, the volunteers, rightly or wrongly, are only going to pay that amount of money if they go themselves. I'm perpetually in the red because of my work abroad. I don't imagine I, or many people, would put themselves in debt to simply give money to a charity.
The relationship building - another thing I've struggled with, I'll admit. But I think it's important to remember that these are children who've been through hell and may never have had anyone care about them or show them affection. The orphanage workers I've met have been genuinely wonderful human beings but they are so busy. They don't have time to have long conversations with the children or organise craft activities for them; the local authorities will not employ enough people to allow them to do anything but work and look after the basics. Also, the children find it amazing that people will come from the other side of the world because they care about them. Yes, they are sad when we go but they are children, they're fickle! When the next group of volunteers come along I'm sure they probably forget all about us!