Hopefully others can provide some insight as I am interested in the same question. But for what its worth, I'll offer my 2 cents.

I currently use my *wood amex for as much as I can, so the first question is, do I want to pay a fee or not. MBNA Elite Rewards has a fee free card, but a) you can only redeem through them, you can't redeem for say, miles in an airline's program and perhaps more importantly b) points expire after 3 years.

Given the fact that the visa/mc isn't my primary spending card, the question then becomes how much are you willing to pay in an annual fee for a card that you won't be using as much (assuming you'd rather use your starwood). In my case, I figured that my usage of my visa/mc would be so seldom that I'd rather not pay a fee and be set on the idea that I probably won't be redeeming anything on the card. just my thoughts.