"Given that each conference spends something like 15% of its income for the retirement of its pastors and teachers, what’s the advantage of letting the conference send in .3% itself?"

Loren, this sounds like "salary packaging", where a percentage of one's income is withheld by the Financial institution running the program, then deposits the amount as tax-free (fringe benefit) income. I'm not sure exactly how it works, but its affective.