They have already established credibility and believability so they make a logical source for your customers and their referrals. It's important that your techs understand the impact billable service has on your customers and your company. You will want to discuss the effects of unpredictable revenue. With no formal arrangement established, such as a service contract, there is no guarantee your customers will ever call you for service. They can call your competitor just as easily as call you.
Revenue becomes unpredictable.
With a billable customer there is no loyalty between you. You can choose to put them on the back burner letting them stay down or send your least trained people and they can switch vendors any time. You should service billable customers at a lower priority than contract customers, after all contracts pay in advance. With unpredictable revenue comes uncertain cash flow for business. If the service department is contributing a substantial share of profit to the company's bottom line then cash flow becomes important for parts purchases, training, salaries and other expenses required to sustain top flight service.