I've asked this question in other forums, but I wanted to make sure I wasn't overlooking something.

Since the Amex Starpoints card has a possible conversion rate of 2.5 miles/$1 charged (charge $10,000 and you get two Starpoints per dollar and then when you transfer the 20,000 points Starpoints adds 5,000 more miles), is there any need for a MR card if you only want airline miles?

You can move Starpoints to more airlines than MR and Starpoints has no annual fee. The only Starpoints drawback I can see is that it takes 30 days to transfer points instead of five for MR. But even that doesn't seem like a problem if you plan ahead.