If you really cared about maximizing your AMEX rebate, you could just buy $6K of savings bonds as soon as you got the card. Then you're guaranteed at least 1.5% back on everything else for the year. Factor in the initial 0% APR for the first six months and anyone could afford to do that.

If you're going to talk about the time value of money, though, for most people--I'm not talking about the typical poster here--getting a check even once a year is better than waiting until they accumulate 25000 miles on an airline affiliated card. This is because you not only have to wait until you get 25000 miles to get your ticket, but you don't realize the value of the ticket until you actually get the ticket issued.