This is wrong to the point of being backwards (I've worked on building these systems). The most demanding clients from an ROI perspective are _always_ the biggest clients, since they're the the only players who can support an employee whose full time job is to care about this ROI. Seriously, look at the features of the ad portals - they're obviously built for major advertisers. Features are prioritized based on how much business they will bring in, thus large advertisers drive the product roadmap.

Small businesses care about ROI in the abstract, but are generally so time strapped just running their business that they barely have time to even set up attribution or measure ROI beyond "I spent this much and it kind of seemed like I got more business".