We believe that the value of a patent portfolio can not be correctly evaluated by mere valuation of each patent of a patent portfolio without consideration of environmental characteristics surronding them.  It is because even gold can glitter when there is light.  Of course, we admit that the value of each patent of the portfolio is affected by legal validity and other traditional "internal" factors, and therefore should also be assessed respectively.  It is because all that glitters is not always gold.  However, we also believe that the value of respective patent can only be properly assessed when it is evaluated as a portion of the portfolio's value consisting of them.

Below is a brief summary on the nature of the major environmental factors.
(From ByeongSeok OH, Patent Value Strategy: Discovering True Value of Patent Management, 2009, Japanese Institute of Invention and Innovation)  

Only commercial use, or market use, of patented technology is prehibited by the patent law. So, value of a patent is dependent on the usefulness of it's technology (patented invention) in the market. However, the market is not simply stable. It changes dynamically and unexpectedly.