The problem is prototypes end up being production code in the real world. Writing tests is about managing risk. You should write some basal level of unit test as you go to validate your logic as you go. That basal level is determined by the team or individual tolerance of risk
Who said anything about production ? Haven't You seen requirements changing even before first prototype is ready? I had a meeting literally today where client's CFO and head accountant throw out my team's week of work because they forgot about key requirements (and this have happened third time this year)
