Less efficient. Sending the intro and letting recipients ignore it is much quicker, instead of going through those 3 steps (compose two emails, wait for responses, compose introduction).
That's true, but it ignores the loss of reputation you get if either recipient considers the intro spammy. And I wouldn't give my email address to anyone I thought would not have the courtesy to ask me before sharing it.
True, it's a question of trust. Also depends on how well you know the person in question, and how confident you are that the intro would be worthwhile.