that said, most evangelists i've known closely for extended periods of time (mostly in the US southern baptist, episcopalian, and US mormon traditions) have not endeavored to convert people in selfish interest, but rather in the (misguided, in IMHO) belief that their souls (the converted) will actually be saved.
according to them, they honestly believe that they are doing something good by "turning people on" to God. this, be it misguided, is altruism - helping people for the sake of THEM.
i acknowledge the fact that my sample size is rather small, and that there really are hell-bent fire-and-brimstone missionaries out there.
furthermore, i've never actually been bothered by missionaries, despite being raised in a relatively "godless" portion of the US. the closest i've been is: once while i was doing ethnography in the southern US - i was approached by a US southern baptist who wanted me to complete a survey about religious beliefs for him. of course i was cautious, but this is what he said, paraphrased from memory:
"look, i'm not looking to convert you. if i could 'convince' you to believe in God in these five minutes, there's nothing that would lead me to believe that the next guy would convince you not to, in the next five minutes. that's simply not on my agenda."
i completed the survey.
IMHO, he did his Christian brothers, himself, and me a great service by saying these words, which were surprisingly reasonable at the time. (i'm not so surprised nowadays.)
finally, i agree with tim in a way, that on the whole, missionaries to a whole hell of a lot more than the average guy, as far as raising the standard of living of the poor. how many of us have spent months in the countryside doing carpentry or plumbing work? do the people need it? yes. have you helped out? you answer.
they've never gotten in my face about my atheism. why should i believe that they're out there hurting people? and why should i believe that if they ARE hurting people, that it is out of selfishness?
show me some concrete evidence that "the foundation of society" is being "torn away" and i may consider your opposition to their activities.
so far as i know, the "foundation of society" is free choice, and so far as i can see, our world is a kaleidoscopic view of that process in action.
the groaning tenor of some of these responses, in my view, is the result of some very unhappy people. perhaps some volunteering, according to whatever -isms you subscribe to, would serve as a salve to your self-centered headiness.