good question and thanks for asking: Platinum Premier is as Pingreeman says, the top three percent of loyal customers. It confers no additional benefits other than a different kind of card, a special phone number where the folks are specially trained for their roles and helpful as well. In addition, at times you will be given special preferential treatment when you check in--though I have found this to be a sometimes thing.
Call the rewards line and see if they can amplify on these comments.
I think Marriott could do a better job improving this level. Most Platinum Premier members got there by spending almost 150 nights - that's Platinum (75 nights) twice over! If you get a ton of incremental benefits going from silver to gold (40 night delta) and gold to platinum (25 night delta), shouldn't Platinum Premier be even more special? Go ahead and limit it to the top 1-2% if it's a cost issue.
To me, being Premier shows real dedication - e.g. sometimes when I can't decide between two potential cities to visit, the presence of a Marriott is the deciding factor.
So in exchange for this much loyalty, I would really like more than just a phone number! To be honest, I don't find the Premier number to be any better than the regular Platinum number.