While I agree that selling something at the highest price you can is the best way to appraise value, I disagree that an up round necessarily increases the equity value of employees (unless the employees can sell off their equity during the round).

To use your orange metaphor, while it is true that if no one else wants the orange the value of the orange is $0, I don't have to sell it to have a good idea of what the price of an orange is. For example, if there are people offering to buy my orange for $8 and I know that a typical bid-ask spread for fruits is %50 of the bid and most transactions occur at midpoint I can be relatively certain that the orange is worth $10 despite no transaction occurring and there being no market for oranges (if there is a generic orange market you have access to then the entire point becomes moot since you can just take the market price).