I was making the opposite assertion. How are classes simpler than prototypes? If you want to add behavior to the proto and the clone, you just add a method to the proto. If you want to add behavior to a class and its instances, you have to do a fair bit more.
Prototypes offer no contract, so it's up to the consumer to ensure that the object received is acceptable. Classes offer a contract, so if an object is an instance, it's guaranteed to work (Liskov, etc.).
Basically, the idea of OOP is information hiding and polymorphism, and prototype-based objects throw both of these concepts away. The result is more complicated code in general.
Ruby Classes don't provide a contract either. You can add and remove methods to your heart's desire.
information hiding is the principle of segregation of design decisions in a computer program that are most likely to change, thus protecting other parts of the program from extensive modification if the design decision is changed
How are prototypes inferior to classes with respect to information hiding? They provide the ability to easily change the implementation while maintaining the interface.
How do prototypes inhibit polymorphism? All of the prototype languages that I know of (Javascript, Io, Self, Lua ...) make it easy to take advantage of polymorphism.