About the brick laying, I'm told that you usually do that poorly if you have no experience (unlike activities like making sawhorses etc. which require little skill). Our local Habitat chapter has experts come in for brick-laying.

In the end, 'voluntourists' are just trying to give what they think they're able to. If they aren't net negatives, I see it as a good thing because they will spend money there, they will bring attention to whatever cause, and other such things. When they're a net negative, though, perhaps it's not such a good idea. It's nice that you want to help, but sometimes what you have to offer isn't what they need.