I'm kind of in the middle on this issue, but leaning a bit more towards the "I wish VIDA and similar programs would be much heavier on their volunteers gaining tech experience rather than surgery experience" camp.
I think there's really 2 issues at hand here. The first being whether or not laypersons should be performing surgery, and the second being whether or not these trips are really meant to benefit the pre-vet volunteers or the animals/communities in the third world.
I can't comment much on the first issue with much authority... since I'm not much different from a pre-vet, seeing as I hardly have a semester down and don't feel that much more knowledgeable about spaying/neutering than when I was a year ago. But legally, at least in CO I'm allowed to start doing so under the supervision of a veterinarian. Even then, I don't feel like it would be kosher for me to go in after a few surgery club wetlabs and surgery skills lab, and have a vet guide me through a spay. However, I think it would be totally ethical for me to observe vets performing spays/neuters and start practicing little snippets at a time. Depending on how many opportunities a first year gets doing that, I can see one slowly doing a neuter him/herself and working up to a spay. So in that sense, I don't think pre-vets are any less capable of "mimicking" spays/neuters under the supervision of a vet than a 1st year vet student like myself.
Regardless of who's doing it though (pre-vet, 1st year, 3rd year, etc...), someone who's performing their very first surgery is going to cause more suffering on the animal by not having a seasoned vet do the surgery. Then what's the difference? I would say it's justification. Unless one fails out of school, a vet student is going to become a veterinarian in a few short years and needs to learn how to perform surgery somehow. The only way to do that is to practice under the guidance of a veterinarian. No matter what, animals are going to have to be subjected to my learning if a vet student is to become a competent vet.
Yes, it's true that many of the pre-vets who go on these trips will eventually become veterinarians (esp those on this forum), but as others have said there's really no screening process for pre-vets. Anyone can designate themselves as one. I think it's justified to practice on animals under supervision in order to become a better vet... but I think it gets a little murky when it's for the "experience."
But like I said, I agree that if a veterinarian is right there, and watches and instructs each step... for the most part, the result will be a live and kicking sterilized animal (in variable amounts of pain)... and given their circumstance in the overpopulated world, it's not the worst thing that could have happened to them. I totally see that point of view, and I think it's a matter of perspective to believe it's ethical or not.
The bigger issue I have is the latter of the two. Like others have said, it's pretty common for a spay/neuter vet to do a good 30 surgeries in a 6 hr span. If you had just 3 veterinarians working 10 hrs a day, they could have knocked out a very conservative 100 surgeries a day (A program at CSU does this, and they knock out well over 700 surgeries in a week). It would take AT LEAST 3-4 great volunteers/vet in order for the volunteers to have animals knocked down, vaccinated, and prepped for the surgeons to have animals constantly at the ready. I'd bet that this would work out even if one vet was designated the non-surgeon who saw appointments throughout the day. Spay/Neuter vets are FAST and it's very difficult to keep up with them... and perhaps the org would be more successful at recruiting more of them if they were the ones doing what they do best, instead of teaching laypersons (which many vets have issues with).
And I understand that when you're a non-profit in the 3rd world, you don't have optimal equipment/protocols... but why on top of that would you add inexperienced hands and prolonged surgery time? The program has perfectly capable veterinarians on site... so why do the volunteers have to perform surgery? That's what I don't get. Because they paid money and flew out of the country? Because without money from the volunteers, the org doesn't have funds for this trip... and 50 animals is better than none? Could this org just not recruit enough paying volunteers if they offered an intensive tech crash course instead of a surgery crash course?