American Express is offering a 30% bonus on transfers of Membership Rewards points to Marriott through December 31. You have to log into Membership Rewards to see the bonus offer.
In general this is a bad deal. Marriott points are worth 7/10ths of a cent apiece, maybe a little less since I valued them two years ago. A 30% bonus means you’re getting 9/10ths of a cent worth of points for each American Express point.
You wouldn’t put spend on a Marriott credit card earning 1 Marriott point per dollar. And you wouldn’t put spend on a Marriott card at 1.3 points per dollar, either. Indeed, Marriott’s co-brand cards generally earn 2 points per dollar for unbonused spend and still that’s not my preference.
Even if you needed to top off your Marriott account I’d avoid doing this. You can buy Marriott points without a bonus for just a little more than just (regular price 1.25 cents apiece).
The general point here is that outside of Chase transfers to Hyatt, conversions of transferrable points into hotel points is simply a bad deal and a 30% transfer bonus doesn’t reverse that calculus. American Express points are just too valuable to transfer to Marriott given the weak transfer ratio.