I agree with both Bogey and BocaBoy about the loss/devaluation of the direct-purchase Marriott Rewards exchange benefit. In our very early years the number of MR Points we'd get for our 3BR SurfWatch Weeks was more than enough to exchange for one of the "Around The World" (or whatever they called it) packages that included airline tickets (not frequent-flyer miles,) 7 nights in a top-tier hotel and a car rental. Our biggest regret is that we didn't take advantage of that option before it disappeared. And of course as the years have gone on there's been increased continuing devaluation in the Bonvoy (formerly Marriott Rewards) program, BUT! consideration has to be given to the fact that there has always had to be reciprocity for MR/Bonvoy Points usage between MVC and Marriott, Int'l (both before and after MVC coming under the Marriott, Int'l umbrella.) It's always appeared to me that MI calls the shots and with them effectively holding the timeshare segment under its thumb, there's not much MVC could do before/after without US paying for it. Sure, I wish it was as valuable a benefit as when we first bought, but I don't necessarily fault MVC for the fact that it's not.
I never had experience with the Rental-By-Marriott program but do understand criticism of it more than that of the MR/Bonvoy program. Rentals by Marriott have become so devalued and such a cash grab for them that I don't think even the prime-est of prime inventory is worth giving to them. I think this devaluation did more to contribute to the disparity between direct-purchase and external resales because it forced owners to establish and use an external rental market, effectively forcing the online timeshare rental/resale market to explode.
On the whole, though, we are still very happy owners and like very much the way our ownership has been enhanced by the introduction of the Destination Club. That's not to say that every owner should feel the same as we do because every owner has their own reasons for purchasing in the first place. Still, I'm always saddened to read that longtime, knowledgeable owners have decided it's time for them to sell because they're unhappy with the company's direction. Always.