For VOLUME, Lenny's right on the money. Things are going the "pay to play" route because people assume that QUANTITY of traffic is the best metric of any campaign's success. Perhaps, once "pay to play" is the standard, the next step will be a standardized measure of ROI? Volume is nice but volume that pays for itself along with producing a profit is even nicer. This is a big problem and will not be fixed quickly since there's many technical/logistical issues re standardization and reporting accuracy.
Regardless, there's other free sources of traffic out there. See:
http://justtraffic.blogspot.com