Very disappointed to learn that Marriott is no longer part of Amex Membership Rewards program. I learned this when I went to book a room. Travelling for business, I am mandated to use my corporate Amex. Any rumblings as to whether Marriott will get back into the MR program. I am a lifetime platinum and am frantically trying to get enough points for next summer's vacation. The Marriott Visa, while I have one, is not an option for my business travel. I also let Marriott know that the Chase customer service as related to the card, is awful.
Also, not sure why, but Hilton Honors gives its top level (VIP Diamond) a 100% bonus instead of 50% for Marriott.
I don't remember exactly when they stopped the deal with AMEX membership rewards, but it's been several years now. Sure was a convienent way to pad my MR points account. AS for getting enough points for an upcoming trip, make sure you're registered for the current 25 nights/50K points promo. And the 3 nights in CY for 10K points promo. Seems like there's always something going on with an opportunity to earn bonus points.
In addition to NUHusker's comments, Marriott does have an offer: Save 5% with American Express. The offer applies to 5 participating brands, all Limited-Service.
Beware, it's not clear what properties participate, I encountered issues using the Search engine at MarriottOpenSavings.com. The skinny is a poor confidence level in the accuracy of the search engine. Rather than waste precious time online, call Platinum Elite.
Reading your post called my attention; as I have never seen an exchange on the AMEX website for points but rather for certificates that weren't that cost effective (If you want me to be honest with you the best trade off for AMEX points is airline miles). AMEX still working with Marriott; you get 5% cash back (If you have Open) at the midclass brands (Fairfield Inn, Courtyard, etc) and if you book meetings and pay with an AMEX (As long as you register) your meeting points will be doubled up to 50,000. As far as the points, I' am a Diamond with Hilton and for as long as I have been with them, I haven't been able to redeem as many nights as I have with Marriott, until now Hilton did not offer any type of bonuses (because I got the email yesterday by the way is 25K after the fourth stay, or double base points until Dec 31) , like the megabonus that Marriott offers and to 40K with hilton Equal 1 night in a category 6 (most nice hotels fall here), 40K with Marriott equal 1 night in a category 8 (there are a handful of hotels here, there is plenty to choose from 5,6,7) and you don't get the 5 night for free. This makes their points to have less value. The current megabonus is only with Visa cards, which I know sucks (I pay with my AMEX too) but not all of the megabonuses are tied to the card that you use.