If you book hotels on ac.com, reservation is actually made through hotels.com which means a $25 fee if you need to cancel, plus you can't earn any additional Marriott or airline points. I did it once many years ago thinking I could double dip. I earned 200 miles by booking on ac.com, but I lost the 500 miles I would have normally earned for a night at Marriott!

If something has changed, please correct me, since I haven't used this in years to book a hotel. Burned once ... never again!