Originally Posted by glitchy

It's much easier to get retroactive mileage credit with a boarding pass than without, which is an annoyance I'm going through now. A friend has also gotten hassled by inbound US c&i ("you're not on the passenger list for that flight, how did you really get here?") and having one was most helpful.

That said, I usually check in online and print at the airport kiosk (or have the agent do it if I'm checking bags), but this thread is making me reconsider. Or at least make sure I have a charge!