For everyday spending, it's certainly not the best choice, primarily because Marriott Rewards points aren't that valuable. But for the categories where it earns 2 points (especially if they don't take AmEx), or spending at Marriott (where it earns 5 points), it's a decent (or even good) value.
I've recently installed a useful iPhone app called Wallaby that keeps track of the different category bonuses (including quarterly promotions like Discover), and that makes it much easier to remember which card to use where.
Starwood AmEx has been my "go to" card for about 10 years now, and I'm glad to see it's at the top of their list.
The other thing it doesn't really factor in is the fringe benefits. An extra elite night for every $3000 spent, the points towards lifetime status, and the annual free certificate, while not much, do increase the value a bit.