Of course Marriott can simply pocket the rent.
Marriott is the actual owner of much of the Trust inventory (unsold Trust points). Marriott is free to rent and pocket the profit from that unsold inventory. In doing so, Marriott's only obligation is to substitute an equivalent amount of inventory (as measured by the points valuation) for the prime week that they claimed and rented. The gotcha for owners is which weeks Marriott claims for itself, and which weeks are substituted.
So far as I can tell, there is nothing in the legal docs that prevents Marriott from scooping up the best weeks for itself. And there is nothing that prohibits Marriott from substituting near-worthless off-season weeks if they want (so long as the total points valuation is the same).
There is ample anecdotal evidence that Marriott is doing exactly that kind of substitution.
One example is cited here:
http://www.tugbbs.com/forums/showthread.php?t=168111
Another example here:
http://tugbbs.com/forums/showthread.php?t=162068
Another example here:
http://tugbbs.com/forums/showthread.php?t=166674
Marriott Vacations Worldwide isn't a non-profit corporation. They have shareholders who expect them to generate a profit. Their fiduciary responsibility would actually prohibit them from returning rental income profit to the trust to reduce maintenance fees - that would be robbing the shareholders.