The basic argument is that whenever the government interferes it causes inefficiency and unintended consequences, therefore it shouldn't interfere. To support a completely laissez faire system you would have to believe two things: 1) that efficiency is the top economic priority, and 2) that all market failures and externalities will work themselves out. If you believe those two things, laissez faire makes sense.
Supporters of laissez faire would almost certainly admit the difference between a perfect, competitive market model on paper and real life, but they would argue that the total costs to society caused by its failures would be less than the total costs of a more interventionist system.