As noted by other posters, you hardly ever need to have the actual card (with the exceptions being Southwest and Hertz #1 Club Gold, I guess) I find it reassuring to have them with me anyway. I always put the cards for the programs I know I'll be using on any one trip in my wallet. But I also put the cards for favorite programs and programs in which I have elite status in my carry on.

I know that AA Reps can also slide your AAdvantage card through a reader on their keyboards to bring up your information. I typically give my card to them when I check in but they rarely use it. They seem to prefer to key in my last name (it's short!) or the record locator number.

A while back I was checking in for an AA flight and the Rep needed to call the AAdvantage department to confirm something. He asked for my AAdvantage card to get the phone number for the AAdvantage desk--because it was easier than looking up the number on the system!

I pride myself on my light packing--but I feel these cards take up very little space and occasionally come in handy. I also carry a list of all of my important numbers on a laminated card--and stored in an internet storage account. I also always bring my passport (and several photocopies) even on domestic trips--because you just never know!