The cards will help alleviate the possibility of inconsistency should you have a worst-case occur that requires investigation into your teaching methods. If you go by the cards, then it's difficult for you to be wrong.
That said, yes they are a good investment. When I started my PADI DM work, the cards were part of my course materials. With SSI, they most definately are.
My PADI Rescue Diver course included the accident management slate, but that was in 2002. As for your personal emergency assistance plan, this is why I stress having your dive log with you. Your dive log should contain the specifics on allergies, emergency contact info, physician, etc.
SSI Instructors have a pages in their ProDiveLog that are designed for emergency assistance plans from a specific dive site. Any site that an SSI instructor teaches at is supposed to have one of these filled out according to the training standards.