Check this out. When searching hotel rates (no discount like AAA or Senior selected), Marriott is now showing comparisons of what other sites are offering.
I did a quick check to see if the the prices on the other sites are accurate. They are. Of course Marriott chooses which rates to show you for the comparison, so this doesn't mean you won't be able to find a better rate elsewhere. I'll be curious to see when booking my next stay if any of the competitor websites have a better price. I'm going guess they won't.
I rarely book anywhere but Marriott and I almost always use a discounted rate so comparing prices to third-party websites isn't likely to give me any useful information. Still, it's something I hadn't noticed before.