I told someone that they should never wear a button down collar to a formal event and he scoffed at me. Thoughts?
Assuming formal = suit & tie or comparable, you're usually right. Some will say to never wear a button down with a tie, but I don't mind it with a blazer or sportscoat because that's generally a less formal look, anyway, especially if you're talking corduroy or tweed and/or a sweater under the jacket. Buttondown collars with a sweater help keep the collar under control so you don't have one collar in and one collar out.
Spread collars are the way to go these days for formal (suit). Really showcases the knot.