The MBP will read the P2 card via the camera. So this is a workflow question and not a what must I have.
The Duel Adapter works well and it gives you 2 major benefits. 1) you can be using the camera while you are downloading another card to your computer. 2) It transfers at least 2x as fast as when using the camera.
The adapter is very reasonably priced at $120. I had one go back but the company did an advanced replacement which was great.