Ebates has 6% cash back right now for Marriott reservations. I just cancelled all my reservations and re-booked them so I would get 6% back!
This is the highest I've seen, its normally 2.5% and sometimes 5%.
If you arent using Ebates, sign up here https://www.ebates.com/r/ASHLEY38090
Since Marriott Rewards points are worth a lot more than 6% of the money spent, this does not look like any sort of bargain. As Platinum, using the Marriott VISA, I get points worth about 20% of what I spend at most Marriott properties. I would not give that up to get a 6% rebate.
Yes, I am platinum as well and use the Marriott Visa also. Ebates is a separate cash back reward. Just click the link above, sign up and then click marriott and after your stay you will get 6% back into your Ebates account which can then be sent to you via paypal or a check in the mail.