It's tempting to solve an easier problem only superficially related to the actual problem.
Another metric of this kind would be stock levels expressed in dollar value or in days of sales. Very easy to cut and show "benefits", especially if you are in long lead time business. Cut them, update your CV and make diagonal jump to another business. Profit :)
Service levels are probably the favorite proxy variable in supply chain. Supply chain textbooks and consultants assume that "finely tuned service levels" automatically translate into better outcomes for the company, while those service levels say very little of substance about the quality of service actually perceived by customers.