Speaking as a relatively successful SEO company, I have never considered offering pay as you go results and never will.
If you beleive in what you are doing and have many testimonials and good case studies, you should not have to prove yourself to others by offering a pay on results service. I do not commit any of my clients to a contract, meaning that they can use us for a couple of months and then move onwards should they wish.
In two years our retention rate has been brilliant, people do leave, but generally come back after a break.
I started with one client and now have 30 or so monthly and there is just no reason why I would work my company on a pay on results basis.
If you know you are doing a good job, people should pay for it.
I would never go to a mechanic and pay them per mile for fixing my car, as they offer skills I do not have.
It is hard enough getting them to pay as it is sometimes