Around the D.C. region, where I do most of my measuring, it seems to me that a non-trivial percentage of runners - the more serious types, obviously - just won't register for a race on a non-certified course unless it is for their own fave charity or is on a XC course. I explain to clients that the cost of certification is easily justified by the additional entry fee revenue that would go missing without the certification. And, if the race is an annual event, the amortized cost of the certification may be less than a couple of entry fees per year. Stating this fact doesn't always convince, but it usually works for me.