I get the obvious benefits, but I don't see what are the downsides of joining Mileage Plus program? Do they require you to apply for credit card? Or you can join by just filling in your data during check in?
If credit card is required and I satisfy conditions, is there anything I should be on a lookout for like yearly fees? If I get credit card I would never use it and just have it so that I am in MileagePlus... but would obviously prefer not to have to apply for credit card at all.