No this does not apply to all cases, the Defender (for the regular user) is overkill plain and simple. It makes the phone at least 4-5 times thicker. I can understand people who need this for their jobs/construction/dust/sandy conditions.
You said it's made of cheap materials. Most all cases are made of silicone/rubber and various plastics. That's why I said your objection is with cases in general, and not the Defender.