What's really important is the integrity of the auditing system which is supposedly supervised by the Audit Bureau of Circulations (ABC). Why the ABC allowed the LA Times to conduct a "simultaneous" audit last September is a question that has not been answered.
http://www.geocities.com/truthmasters/watch04-4.html#1031
Apart from public confidence in the system, this information is of particular importance to advertisers since the rates they pay are dependent on circulation numbers.