Yes there is, depending on the type of job he's hired for with the vouchers, and whether the voucher covers parts as well.

Not so: In my scenario, the labor for any job would eat the entire voucher. (I'm assuming he's got a "limit 1 per customer" clause in the small print).

Sure, you can pick a job like that. But similarly, the voucher recipients could pick jobs that cripple the plumber financially.

Perhaps I'm not following you completely. Please give an example where that could happen, given the scenario I outlined above.