depends on whether change is desired or needed. In most cases, the existing culture and religion were doing OK. The missionaries and their hangers-on bring trinkets, new diseases, and an alien religion with them. All too often, the disruption caused by those new things ends up destroying the existing culture over time.
In sum, typically it is a negative exchange. When the Spanish Catholics came to the Americas, it only took a couple hundred years for the existing cultures to be completely wiped out. They brought soldiers with them, enslaved the indigenous peoples, converted them to Christianity by force, changed their diets, their way of life, gave them European diseases, and then discarded those peoples.
The same things happened almost everywhere Christianity came through missionaries. The missionaries were the first intruders, but were followed by the exploiters, who quickly forced the indigenous people to give them whatever natural resources they had. Slavery, murder, genocide, and displacement were the rewards for those people.
So, I can't see any benefit to missionaries, really. Their promises were false, but their exploitation of indigenous peoples were very effective, typically wiping out entire cultures and so allowing their fellow countrymen to come in and steal their lands.
Ugly crap all around.
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