If you are talking about receiving your bed preference only 20% of the time as a Plat, I find that very hard to believe. I'm a LT Plat with 1000's of nights at Marriotts (and most of them as a Gold/Plat.) I've very rarely over the last decade NOT received my reserved bed-type. And as I stated earlier, all I had to do was mention the elite guarantee. Once I did that, they were able to find me an appropriate room.

Note that if a hotel offers you an "upgrade" that upgraded room doesn't have to be the requested bed-type. But on that token, you don't have to accept the upgrade. You can always say I want the exact room type I booked (which would then mean they have to honor your bed type.)