I think partnership with an international brand sets them apart from a lot of other independent options, and may drive higher revenue from international tourists. Certainly the rates that they are asking for July/August are quite a bit higher than comparable properties elsewhere on the island - if Marriott's helping them to get those rates, then the partnership may make sense.

And if they're full, points redemptions shouldn't hurt them too badly at all, as they're reimbursed at/near ADR, if I'm not mistaken.