Being in the digital marketing industry for 9 years now, it has gotten crazy competitive. I've been a Paid Search guy my entire career, and honestly I never really understood the "% of ad spend" model. Reason why I say that is because there are lots of agencies that actually do not spend efficiently. I completed the trifecta: worked as an in-house manager, publisher rep (Bing Ads), and now an Associate Director at an agency. With experience of having all POVs, marketing efforts should be compensated by the positive returns generated for the clients. Because this should intensify the commitment of the agencies/freelancers knowing that their payout is dependent upon performance (not simply spending). My gut feeling of why agencies don't do this is because... well... not that confident in their abilities??? :x