Actually, I think both your sister's best friend and you are both partly right. It is best to concentrate flight activity on as few accounts as possible to maximize elite status, but I believe it is prudent to maintain accounts with all the major airlines if one is wanting to maximize their earnings in this FF game. Different promotions from different airlines offer many opportunities. For example, in the past ten years I have accumulated mileage and redeemed travel awards in DL, US, WN, AA, BA, UA, CO, NW, YX, and FL. I could have easily not used several of those, but various promotions, bonuses, and other offers made it best to accumulate miles and points in each program at different times. So basically, for all non-flight activity I take advantage of whatever rental car, hotel, dining (iDine offers can vary significantly between partners!) or other offer is most lucrative and build up my balances in each account knowing that eventually I'll have enough to redeem for something when the time comes. A great example of how this proved useful is my CO account ... I hardly ever use it, but have accumulated miles over the past 10 years here and there from different promotions and other activities that were lucrative at the time. A few years ago there was a great full-Y MSP-LGW fare available that I bought tickets for my wife and I, and conveniently enough I had accumulated some of the miles I needed to upgrade those tickets already. I didn't know what I would need them for when I accumulated them, at the time it made sense to take them in CO, and I knew they would be useful some day, and sure enough they were. I topped up the account from AMEX, and I was set for my upgrades without having to pull all the miles I needed from AMEX.