Service trips to third world countries - useless or not?

I was in the Dallas/Fort Worth airport yesterday, and saw two large groups of teenagers wearing t-shirts that identified them as traveling with a church group on a service trip. One of the service trips was headed towards Costa Rica, and the other, Haiti.

My immediate reaction to group such as these is - how could they possibly do anything useful? They almost certainly don't speak the language, or have any skills that could come in handy. So how are they serving?  That's really the only part that bugs me - that they call it a service trip. Call it a homestay, language learning, something like that - that's fine. I think it's very good for pampered teens to see other parts of the world that aren't so wealthy.

But calling it "service" is false, because they're not doing useful work. I've heard of orphanages being painted, and repainted, and repainted again, all because groups such as these need service projects to work on, and the orphanage receives a substantial donation in exchange for hosting these "service" trips. Also, mostly on these types of service trips, the teens stay together in groups - easier for their chaperons to manage, but they interact much less with the locals.

Here's a couple links on these kinds of trips:
http://www.theguardian.com/commentisfree/2010/nov/14/orphans-cambodia-aids-holidays-madonna
http://pippabiddle.com/2014/02/18/the-problem-with-little-white-girls-and-boys/

In the interest of full disclosure - I was on a trip somewhat similar to this many years ago, in Nicaragua. It was in the 1980's, I was in college, and this was a "solidarity" type group, against the US-supported Contra rebels. I wouldn't have considered myself a socialist at that time, but I was far more liberal than I am now, and didn't have nearly as much skepticism (cynicism?) about government in general. The trip was 6 weeks, and it was the first time I'd been in a third world country. It was incredibly interesting, I had a lot of memorable experiences, and met people that I never would have been exposed to without this program.

For part of the trip, we were meant to be helping out at a Habitat for Humanity project. That was the service component of the trip. They were organizing and funding a project to build low cost cinder block homes in the area we were headed to, and all of us in the group (about 6) "helped". The helping part was truly ridiculous. There was a group of locals who knew what they were doing, mixing cement. They had an interesting trick to make the shoveling easier. One guy would actually be holding the shovel, and another would yank at a string tied to the shovel, right where the wood handle meets the metal, to make it easier to lift. They had to be very much in the right rhythm to make it work. Perhaps they just didn't have enough shovels.

Anyway, us volunteers had never mixed cement, and also had very little experience with shovels. We were entertaining to watch, I'm sure, but pretty close to useless.