I try to accumulate miles on two airlines. This allows me to alternate mile accruing credit cards on an annual basis (and thus get a 5000 mile sign up bonus each year.) It also allows me to dine at my favorite Dining for Miles restaurants twice a month (and receive miles for each airline.)
It also gives me flexibility in deciding how I want to redeem my miles. I like my NW account because of its low redemption amount to Asia (60000). I like my AA account because of the different partner award amounts. (eg - 40000 on Swiss Air to Morocco, because it is categorized as part of Europe).
[This message has been edited by DC_flyer (edited 03-28-2003).]