Dave has it right!
The great thing about using the motors cavitation plate is that it is that it is STRONG. The ladders that hang depend on the fasteners that hold it to the transom, and the fasteners that hold it together. Thin wall tubing is iffy, if you are over 180 lbs.
getting into the boat by yourself, with no one to help you, is tough, ladder or not, especially if you have just fallen in.