" A humanitarian could only say 'These people are beneficial!" than he would immediately realize, "And Guatemala needs them more than us. Send them back.' "

Faulty logic. One of the biggest reasons for supporting liberalized immigration is that it lets people move to the areas where they are most productive. People in Guatemala become massively more productive when they move to the USA, which earns them higher wages and makes them better off (along with others who benefit from more production).

This website itself endorses that reasoning:

http://openborders.info/place-premium/

What you're talking about is the more limited concept of "brain drain". IMO its practical importance is a lot more questionable than people think. It's a small factor compared to the global benefits of liberalizing migration.