I have been told by several reps that Plats will be allowed cancellation if reservation is made through Marriott. I have successfully done this 3-4 times,
That said I do not think it is a completely 'official' policy as franchise properties have the right to refuse. A call from Marriott directly is certainly helpful in this regard.
Overall I HATE the one day cancel policy. Even with Plat policy I do not like calling hat in hand to ask for a cancellation. As a result on several occasions have held off reserving (checking availability day before or in morning) only to find when I was sure of my lodging needs that there was limited availability.
Also have been caught where rates increased.

As stated Plat is great for addtl. points but if one policy would cause me to leave the fold the one day cancel is it.