Do polymorphism or conditionals promote better design?

Question

I recently stumbled across this entry in the google testing blog about guidelines for writing more testable code. I was in agreement with the author until this point:

Answer 1

Polymorphism is one of the corner stones of OO and certainly is very useful. By dividing concerns over multiple classes you create isolated and testable units. So instead of doing a switch...case where you call methods on several different types or implemenations you create a unified interface, having multiple implementations. When you need to add an implementation, you do not need to modify the clients, as is the case with switch...case. Very important as this helps to avoid regression.

You can also simplify your client algorithm by dealing with just one type : the interface.

Very important to me is that polymorphism is best used with a pure interface/implementation pattern ( like the venerable Shape <- Circle etc... ) . You can also have polymorphism in concrete classes with template-methods ( aka hooks ), but its effectiveness decreases as complexity increases.

Polymorphism is the foundation on which our company's codebase is built, so I consider it very practical.