Prototyping vs Designing

Long ago there was a question raised on habrahabr.ru about a balance between detailed designing and prototyping of a new product.
From one side prototype allows you to:

• get understanding of technology;
• feel how something similar to a final product works and this helps to clarify requirements;
• make client happy by a visualized result.

From the other side:

Expanding The Iron Triange

As I have mentioned in my previous post I don't presume "The Iron Triangle" as a right approach to desribe project parameters. Instead of this I prefer to consider four parameters: scope of work, project duration, monthly cost and quality.
These are, roughly, parameters that can be discussed with a client at the moment of forming a vision of project. If you fix two parameters then the other two will depend on each other, if you fix three then the forth will be defined automatically.

The Iron Triangle

I've met a lot of definitions of "The Iron Triangle", i.e. "Quick, cheap and with high quality - choose two" or "Cost, time and scope are related, but quality is not an option". Different people put parameters on tops or on sides of the triangle...
To my mind this caused by the fact that the iron triangle simply does not exist.