Quite true, but often it comes down to a trade off between trust and worst quality data.  If you believe that your counter-party in a deal has an incentive to withhold some of the great data they have from you in strategic ways, then it's probably a good idea to just assume they are withholding key data. A somewhat more independent appraiser, even if flawed, is likely to be better than the one you must assume, a priori, is lying to you.