You hear that, and it sounds logical, but when I have compared in the past - I found little truth to the claim. Since you brought it up tho, I ran a new comparison. You may not agree with my cheap taste in hotels, but I like Hotel Mary Carmen, half block from the square, their yard turtles begging for treats, etc. I tried their site, but their reservations feature didn't work.
Ok, so I tried Suites Colonial at Hotels.com vs their site: $44 at the booking site vs $48 direct with hotel. FAIL
Alrighty, try Casa Mexicana. And I am being careful to select their home pages and not fall for google listings that look similar: $65 at booking site vs $72 at their home site. FAIL
I think the hotels have to contract with the sites to not quote better prices on their own pages than offered thru the booking sites.
Then too, I like the 10% rewards at Hotels.com and their satisfaction guarantees, which I have claimed at times when I was disappointed with stays, received, and used at different properties on new travels.
Ok, try Casa Mexicana: