europeforvisitors - 1:47 pm on Jun 29, 2006 (gmt 0)
One thing to keep in mind: Different advertisers have different objectives, and not all advertisers are looking for e-commerce transactions. Many (especially in big-ticket categories) are looking for leads that can be developed into sales the old-fashioned way, through personal contact, service, and negotiations. Those advertisers are likely to prefer straight CPC pricing, while a transaction-oriented advertiser (such as an automated hotel-booking service) may have reason to prefer CPA. So, from Google's and the advertiser's point of view, it makes sense to offer pricing options and not just a one-size-fits-all pricing method.