1. Are you asking only about Plaza lounges or about all Priority Pass contract lounges?
2. Why is it important to be able to enter with the Platinum Card itself rather than with the Priority Pass card?
I am asking about Plaza Premium lounges. I already know the answer that the AMEX Platinum card will not get one into a Priority Pass lounge - one needs a Priority Pass card for that (which Canadian AMEX Platinum cardholders get, as one of the AMEX card amenities). Of course, the Canadian Plaza Premium lounges are also priority pass lounges - so one can use either card in those lounges.
There are at least 2 reasons why I would prefer to go into a Plaza Premium lounge with an AMEX card (if allowed), over a Priority Pass card.
Depending upon the type of Priority type card, for the cardholder may or may not have to pay the entrance fee (US$27) for himself/herself, and also for the companions. My AMEX Platinum card associated Priority Pass card allows me and one companion free access, but I need to pay for all subsequent companions $27. The AMEX Platinum card allows a family of 4 to go in free - 2 parents/2 children 18 years of age or under (I can not remember if another companion in addition to the 4 people can also come in free, or whether the free companion rule is limited to the cardholder if he/she does not bring in family).
In the Canadian Plaza Premium lounges, the agent just looks at the AMEX card and boarding passes, records the number - and one can go in (the customer does not sign any receipt). For the Priority Pass card, the process is slightly more complex - the agent swipes the card (and looks at the boarding passes), and the customer has to sign a receipt. So it might take an extra minute or two for the process with the Priority Pass card - you might think what is a few extra moments for the customer waiting at the desk waiting for the card to be process - but even those few extra mements are precious and which can be better used enjoying the lounge.