I understand why goodwill should be deducted from assets in computing NBV, but why deduct patents, for example, which may have fairly specific cash values?
My take is that it is because there are a lot of estimations with the valuation of intangible assets. For example a specific drug patent certaintly has some value to a company but may be worth something completely different to another company and therefor would not be able to reasonably sell the asset at book or fair value. Thus it is not of the similar quality compared to machinary, cash, or other assets that are comparably valued by others.
Thats the way I view it, but I may be completely wrong.
Another reason why intangibles get eliminated is that they are usually very hard to compare accross companies.
A firm that grows through acquisitions will generally have numerous intangibles recognised on its balance sheet (other than goodwill), such as brands, customer relationship value which are identified upon acquisition.
On the other hand, the balance sheet of a company that grows organically will not feature such assets, which does not mean lack of brand value or strong relationships with customers.
So, it is often easier to eliminate these assets from the balance sheet so as to achieve comparability.
On a related note, you DO consider value of core non operating assets/liabilities like land held for investment and pension plan obligation/surplus when valuing a firm. More details in L2 curriculum :).