Many of us work for employers with formal travel policies. I can and do fly AA when the AA fare isn't markedly higher than their apples-apples competition (DL, UA, US, NW, etc but not WN, Airtran, etc). Sometimes that means a very low fare on AA. Even before 9/11, AA was losing a lot of my business because they wouldn't match the competition on price, but in the last few months AA has been much more aggressive on price. If AA yanks Aadvantage accrual or upgrades away from me in those instances, they would effectively destroy Aadvantage for me. I haven't bought a full-fare Y ticket in over a year. I haven't had to. Some AA competitor is always willing to discount Y on the routes I fly, even as late as three days before departure.

DL hasn't offered benefits on their low fare classes for a while. Read over on the DL board about the impact.