It doesn't show up when you search online, but after you click on a rate, there will be several tabs to choose from (Standard Rates, Packages, etc.). One of these is "Marriott Rewards Offers." It is almost always grayed out, and even when it is active, sometimes the rates are actually higher than standard rates!

But once in a great while, the rate is actually really good.

As an example, look at the Blackstone in Chicago for March 3-6. Standard rate is $149 for the first night, going up to $189 for the last two nights. But the Rewards Rate is $134, and it locks in for all three nights.