IP Finance blog tells me an interesting episode on IP valuation. According to the above article, value of a patent was significantly deducted b/c it hasn't been tested by litigation. Read the above article for more detail.
As the article says, if the practice is industry-wise, the practice encourages litigation (for (in)validity). I assume that the litigation referred to in the article is litigation for validity/invalidity.
Second, sound patents aren't litigated. Litigation occurs where patents are not soundly grounded. This is an irony of the value-deduction practice.
Third, the practice shows the quality of patent examination of the USPTO.